Quoted from KingDaddy:
I would focus on Bally/Williams DMD's and System 11 games. Also, go for titles that are higher-rated - usually they are for good reason and are easier to resell. Diagnose their conditions well; understand what you can fix/restore and what you can't - very important. If you follow these guidelines, you can't lose - you'll always get your money back and can try different games out.
Two provisos re getting your money back when you sell. First, this assumes you didn't pay above market price (which can be tricky to figure out and tough to achieve if you want a certain title and are set on having it within a month or two). Second, this assumes you either don't spend much repairing/modding the game, or do so with an eye toward resale value. Many do these things and make money (or at least don't lose money) when they sell. Others tweak and mod the heck out of their games but lose a bit when they sell. Nothing wrong with either approach, only issue is if you pump a bunch of money into the game and expect to get it all back on resale you may be dissapointed.