Well it could completely put most of the PC developers off and force them onto console, as piracy is a lot to contend with let alone preowned PC titles ontop of that. With the insane deals that are going on these days, why anyone would want to sell a game on I have no idea. So somebody could pick for example Skyrim up for £10 and then sell it on after a deal ends for £25+? Fail logic.
Good example would be Command & Conquer, Tiberium Wars, Twilight and Red Alert 3 were all £4 each, £6 or so for the Tiberium Wars & Red Alert with the expansions. People who pick those up could resell them now the deals done for anything up to the original price (downloads don't suffer deterioration afterall).