This isn’t a very useful guideline. First of all, it requires at least some familiarity with the game to even know what that means, and second, there are plenty of multilands which are either so terrible that nobody would want them, or good enough to be useful but still common enough to be relatively cheap.
Do you have a better one?
I still maintain it’s useful. The amount you need to learn about the game is very limited. The cards say “Land” on them, and learning what the mana symbols look like takes 10 seconds. It’s certainly easier than looking each card title up on a buylist.
It’s true that there are many dual lands that are valueless, but the same is true of rare cards “orange symbol”. The average dual land is worth more than the average rare card, so as a first-pass filter, I think this suggestion is as good as as the previous.
Another good filter, if you’re looking at a random pile of cards: Cards that there are many duplicate copies of are not likely to be worth something. Sure, it could be that you’ve found some uber-collector who has 30 copies of some valuable cards, but in almost every case, if there are a bunch of copies, it’s a low value commonly occurring card.
Re-reading the thread, it looks like the OP was the original owner, and presumably used to play. So another good filter is to just thumb through and pick out any cards that you remembered always being happy to draw. Ignore most of the creatures you find this way (modern players expect much, much more from their creatures than we used to), but the rest might be good candidates for looking up individually.
If you only have 500, it’s not that hard to take a few good pictures and post them. Stack them so the name is clearly visible and fit maybe 4 stacks of 25 into a good hi-res picture, and stick those in an imgur album for us to take a gander at.
Fifth Edition’s symbol wasn’t made up retroactively like the others. It was actually used in printing the simplified Chinese language cards at the time that language was added to the game(sometime between the English 5th ed. printing and the release of “Classic” 6th ed). So back in the day you could have two Fifth Ed cards, one English, one Chinese, and one had an expansion symbol and the other didn’t.
Enjoy,
Steven