First go through them on your own. You'll be able to pick out the ones that may be worth something just by the name. Then looks for typo's, misprints, blatent errors(ex. Barry Bonds is the picture, then Craig Biggo is the stats on the back).
If you don't take these minor precautions you WILL get screwed on the deal. You probably have at least 1 card out of the whole bunch that will be worth maybe $300-500 and maybe a couple averaging $50-100. You grew up with a lot of good athletes that became super stars and prolly have a few of their rookie cards. If you just took the box of cards into a dealer, he would just give you $50-100 hoping you don't care and he'll score off of your stupidity.
Dealers love idiots like that, how do you think they make their money? Not buying packets of cards thats forsure. They score on idiots that stroll in wanting to rid themselves of a box of cards and come across a rare Mickey Mantle or some superstars rookie card or the big hit is having a misprinted card like the example I gave you of whoever. Those are the rarest. 10 of a couple hundred thousand actually make it into the packets and you may have 1 of them.
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