Is there an optimum time of the year for buying a second hand car from a dealership? When you are most likely to be able to get the best deals.
End of the month is always said to be best, because most dealers run month-by-month sales contests and everybody is wathing their gross totals.
Also, for used cars the best time is when new car sales are briskest.
This is at the start of a model year, and the end of model year, and also when the manufacturers are giving rebates or low-cost financing.
At these times space is at a premium on the lot and tradeins are coming on a regular basis. Owners want to get rid of the glut, so sheer numbers are in your favor, plus, since the tradeins are fresh they are more likely to have good cars that have only been shown a few times and the first one to spot them gets the best deal.
I’ve heard January 1st from two different people, but neither one could explain WHY.
When you don’t need one. 
This is more than you’re asking for, but here goes:
Generally, last summer is best for new cars as the next model year is rapidly approaching and the dealerships need to move the current model out of the supply chain. Likewise, you can get deals on unpopular cars (Like Suburbans when gas is $3 a gal.)
All these new car sales have really dumped used car values on their ear. So much so that our Car broker friend won’t bother with NADA trade-in values for SUV’s at the moment. They’re REALLY not worth that suggested amount in this market.
The best PLACE to buy a vehicle from is a small dealership, in a small town, several hundred miles from the big city. Those dealerships have to sell every car they own to keep in business. When you walk into a dealership owned by a retired football player in a Big Town, they already have you at a disadvantage. If you’re too savvy, and you know the math, and you know what the vehicles are worth, they can let you walk knowing the next 7 buyers in the door aren’t as smrt.
The best vehicle to buy is typically a two year lease return. The lessee took the big hit on depreciation, there’s still some warrantee left, and it almost still smells new.
How do you ensure the car is in good working order from a boondonks dealership. They aren’t likely to offer any waranty are they?
If it’s a newer car that still has a factory warrantee left, you can get it fixed at any dealership.
Anytime is good. Really.
Do your research. Check out Edmunds, Kelly’s Blue Book, all the other online auto sites, the local newspaper for similar vehicles, and everything and anything else you can think of. Establish your price. Write it down. Walk in to dealership with that price in mind and in hand, mindful that they will try to tell you they can’t meet that price. They’ll try all sorts of tricks and what-not. Don’t budge. You have your price. Tell them flat out that you will walk if they don’t meet your price— and mean it. Make an appointment with your salesperson. Tell them you have another appt with another dealer tomorrow, but you’ll cancel that one if you get what you want today. Turn the tables on the salespeople, tell them they can meet your price and get a sure sale today, quickly, or spend the rest of the day on the lot and perhaps not get anything at all.
If they balk, you walk.
Watch out for financing schemes too. Make sure you have that detailed in your head too. $$$ at %% for xx months. Do not get suckered into the "what are the payments " mentality.
If you are doing a trade… keep that as a separate deal. Trade-ins are separate transactions. If you must combine the two, do same research on your trade-in as you did for the car you want to purchase. What is it really -worth-.? Write it down. Know it and stick to it.