You are absolutely correct. My bad.
Could you maintain a safety deposit box, or brokerage account, for decades without checking up on it? I could imagine stashing the comic book or other item in a safety deposit box, but can you pay for 50 years up front, and NEVER be contacted? Or if you go back to buy some stock low. Can you put it in a brokerage acct, asking them to just keep reinvesting any profits, and to NOT contact you for 50 years?
Or, to contact the “you” of the future, and not this guy who is putting money into a fund in the early 20th century (or earlier?)
I imagine one way to do this is to find some law firm that has also lasted centuries (there are a few who can trace their founding to the 1700s), and have them operate as your fiduciary for purposes of managing the account (you’d of course have given them strict instructions), with the understanding that you’ll eventually go to that firm in the present day and claim your inheritance.
Eh, you know, nothing in the OP says it’s a one-time round trip. Almost all of the latter issues in the thread, can be fixed by showing up periodically (possibly after visiting someone for a good makeup job in the latter years!) and checking in. Especially prior to wide-spread railroads, travel took a LOT of time, and being out of touch for years would be the norm, as long as you had reputable representatives to manage things for you.
Set things up, make several visits in the first few years, then mention you’ll be travelling abroad and more difficult to reach, drop off the number of visits, and conduct a great deal of business and requests via mail or telegram. Sure, lots more trips, more record keeping on your behalf, but this is about making money! Expect to do SOME work for it. 
For all these issues, you ask your grandfather to come with you. You go to the bank, or whatever secure storage you’re using, every thirty years or so, and the “old man” registers a will that gives everything to the “young man”. Wash, rinse and repeat until you reach an age in which you can turn it into useful cash, say when you’re in your teen years. Perfectly legal(ish), and prior to modern identification systems, near impossible to detect.
Since we are inventing time travel, what if when you change the past it splits off a new universe, but when you return to the future you return to your own timeline?
If that’s the case, you can change whatever you want in the past - kill your grandpa, invest in Apple, whatever, but when you go back to the future it will be as if you never left. You just killed your grandpa in an alternate timeline.
And if you went back again, you’d still be going back to the past in the current timeline, which means none of your interventions happened. If you went back to the exact same time you went back before, you wouldn’t see yourself because each time you go back it spins off a new universe.
I don’t think I’ve ever seen that kind of time travel described in a story, but it seems as reasonable as any.
It’s similar to the time travel in Roger Zelazny’s Roadmarks.
Time travel involves driving along a road only certain people can access, and changing a time line creates a new branch of the road. You can go back and forth, choosing different off-ramps to see what happened. If an off-ramp becomes popular, it grows in size, and if not, it gradually withers away.
Another SF story: “Compound Interest” by Mack Reynolds.
Also a major plot point in Tom Holt’s “Flying Dutch.” A major character takes out a life insurance policy in the 1600s. He then becomes immortal. Cut to Present Day, when the accrued interest on the policy is multiple times the global GNP.
And that’s why modern day whole life insurance policies automatically pay out at age 120.
Don’t bother with big jumps. Learn about investing in futures and then repeatedly jump a year forward (or back) to reap the rewards. Don’t tell your friends about your time machine.
Refer to the 2004 documentary below for hints & possible dangers:
Heinlein’s “The Door into Summer” touched on this. The character is sent to the future as a means of dispatching him by nefarious people using cryogenic sleep. When he wakes up a few decdes later, he finds out time travel has been discovered, so he travels back in time to exact his revenge.
In the future gold is cheap, so he wraps gold wires around his waist. He then sells it in the past and raises enough money to invest in a bunch of stocks he knows are going to go through the roof. He buys an insurance policy, and has the insurance company manage the investments. He deals with his enemies, then he takes the long sleep again to get back to the future and collect the money.
It’s a great story, minus the slighly creepy relationship with his step-niece.
Bah! If I wanted to do work, I wouldn’t have spent years of my life designing and building this damn time machine!
Acquire some paper currency or coin which is old, but not so old that it’s worth significantly more than face value.
Travel back to when that currency was less old and spend it on small purchases, acquiring older currency as change.
Repeat until you can acquire gold coins at face value.
Well then, go back and tell yourself that! I mean, all of the contradiction issues have been resolved, although arguably, creating a time machine might count as one of the big events you can’t change.
Still, make it easy on yourself. Obviously you had at least some money to research and build the machine. Go back to yourself and advise of a dozen cheap stocks that have improved in value that you can invest a few hundred to a few thousand in a decade ago. Return to present.
You now have more money, from work your prior self did. Keep making that bugger work for you!
Then go ahead a few months/years, figure out which stocks are going to go big (use a public library for access, which may not be available to far into the future the way things are going, so keep it pretty close), return to the present, and invest your newly increased funds. Voila, more money.
Less work than trying to figure a way to find representatives and safe deposits in the past. Which was filthy, with human and animal poop all over the place, no good Toilet Paper, and hot and cold running plagues. Who needs it? And if it turns out the future is going to become a dystopian hellhole, you might want to just keep rewinding a few good years of the present and enjoy the wine, women (or men, I don’t judge) and song between the filth ridden past and roast-rat over a literal dumpster fire of the future!
Easy enough. Pop back to 2010 when bitcoin was a penny and buy however many thousands were available. You probably already had a PayPal account then.
Next stop, late November 2021 when the price was high 60k.
Now, if you want a first edition Superman, a roll of gold doubloons, or any other date checkable collectable. Just put them in time capsules and bury them somewhere that won’t be developed until after whatever time period you decide to settle in.
Wouldn’t it be sufficient to bring back gold with you? I’d suspect that for much of the past couple of millenia, you could find somewhere to convert gold into currency.
I my experience time machines never work twice. Pretty rare that they work at all really. But if you get one to send you through time and back once you should take a trip to the recent past with some lottery numbers or sports scores or some knowledge of an investment that will soar in value. You may not be able to get back to the present so you don’t want to go back to a time you won’t like living in to find out that quantum uncertainty and multiple universes don’t produce the same numbers. You don’t want to go far into the future to find yourself in a world you aren’t comfortable in, may have no way to survive based on your 21st century skills and knowledge, and no way to get back. If you do make it back and try it again, maybe then work out some long term scheme to earn interest over hundreds of years or reap the rewards from inventions from the future.
Your response is totally responsible and fair, given the majority of time travel stories. BUT - I don’t think they apply much to the OP, who seems to be leaning towards the comparatively (!) fool proof option of Bill and Ted (or Back to the Future but with fewer emergencies) in the original scenario. After all, they’ve figured out the contradictions and apparently confirmed the inability to make significant changes in at least two separate timeframes.
But agreed, one of the things you want to pack in your TimeBag is a supply of emergency knowledge, currency, and survival supplies for the ironic or plot driven fuckup that seems inevitable. And that’s why in my later examples, I went with the easier option of staying -close- to the present. If you get stuck, better to be in a near-past or near-future where you’d hopefully be comfortable living in a style and society that you’re comfortable with.