In that case, collectors who seek to sell aren't going to be successful if people knew not to spend on overpriced items which would mean being a seller is a bad investment.
Not so if you can get your stuff for real cheap and know your game.
In my case, I buy, as you all know, lots in which sometimes I grab cars that are really sought after, for practically peanuts. I grabbed a lot the other day that included an original Real Rider Cobra in blue, along with other 5 cars, for like 7 bucks, shipped. I could sell that car for like 20-30 bucks over here because it is really hard to find here, has Real Rider tires which had a very VERY low supply over here in Mexico back in the 80s, and is a pretty popular casting in the perfect color combo.
Yet as I got it for like $1.50, I can offer it at a lower price, let's say $17 because yes, I got it for cheap but I'm not going to give it away, but it's still a good price. I made back more than 100% of the investment of the car, the buyer was happy as could be, and will come back because he knows I don't want to get rich selling cars; I still do a killing profit and help the market in the process by lowering the prices, because I know some sombiches that would sell that thing in 50 bucks.
And so it goes, you have to do some homework about what is sought after and what you can get for cheap. There are people out there that also know what is valuable and what not and will immediately laugh in your face if you try to do something stupid, but that will also reognize a fair price when they see it.