If the ruling is upheld, what happens next is going to depend on the licensing agreements written for each game sold. Merchandise like clothes and books are covered under "first sale", but a different set of rules applies to software because computer programs are sold with licensing agreements. The agreement can be written to include a clause prohibiting resale of the product.
So if the ruling is upheld, game companies can write new licensing agreements for their games to prevent them from being resold.
I have a friend in the game industry, and we haven't talked about this subject, but he seems in favor of buying originals because developers don't make money from used sales and it's hard for small companies to stay in business. Also, Gamestop is a major retailer, and I think they sometimes push their weight around, so cutting off one source of income from them might be a way of fighting back on the part of the game companies.
If I were to call it right now, based on gut instinct and no prior research, I would say that banning the resale of games is a questionable move. Even if it were just to hit back at Gamestop, it would also apply to EBay sales made by individual consumers. If I were to hazard a guess, I don't think such measures pay off in the long run; I suspect that they actually hurt the market. For them to even resort to such measures, I wonder if the industry is reaching a plateau. But then, I speak from ignorance here.