Might actually find more games at more yardsales!
On the flipside though, I'm afraid that many people who don't know any better will assume that any old video game will be worth a boat load. You'll see people asking $30 for Mario/Duck Hunt because it's "old."
People already do that. If nothing else, it will normalize pricing in a lot of cases because they can look it up on gamestop, or see how much gamestop is asking for it. Now, it gamestops pricing is CRAZY high, that will suck. If it's not... Great!
People who INVEST in retro games should be scared.. Gamestop will drive a lot of the pricing. And you can almost bet they will be LOW for a while, then adjusted over time. Should definitely shake up the environment over the "pricecharting" site. The nice part might be some new interest in Retro games over all which will help create a more fluid market, vs. the stagnant one you see right now. Might actually find more games at more yardsales! 
Really? I feel the opposite and eventually there would be a lot less old games at flea markets
Why? Right now you have just a few game shops and some e-bay markets for older games. Basically, it's in the hands of a very limited market. Once you expand that out to Gamestop, you're going to have a lot more people who purchase and play these older titles. Do you think they will be willing to sell them back to Gamestop in most cases? I'd wager they'll try to sell them on their own first, so Craigslist, Yard Sales and flea markets should at least see a jump in some people selling. I can't even count how many times I've seen people post stuff for sale because they have buyers remorse after playing it for a few weeks, well past the 7day return at Gamestop and they aren't willing to take 1/10th of what they just paid gamestop for them to take their merchandise back.
Nobody's gonna trade their old games in at GameStop. This idea is DOA.
You'd be shocked how many people still try to.. I was in one the other day and someone walked in with a Sega Genesis and a Super Nintendo and they turned them away. Happens even MORE often with Gamecube and XBOX stuff.
I don't know if this will be good or bad overall, but it should at least shake stuff up a little, which won't be a bad thing, all my local shops have some CRAZY high pricing so they will feel this the most, I have a feeling that most decent retroshops won't have any issues, but the price gougers will suffer.