So think that through - nobody's going to do massive discount sales anymore, to stop people stockpiling games to sell at a profit later on. But you might be prepared to pay more for a game if you know you can get half back when you're finished, so you still buy the game. Maybe.
That doesn't make any sense. If anything, 75% discounts will be out, but in exchange we will get lower initial prices, rewarding us in similar average prices over the year with the ability to sell the games we don't play any more. It will only encourage sales from publishers + extra DLC codes for those who buy new games. In effect, if you buy a used game, a small administration fee should be split between Valve and the publisher, if you like the game, you will support the developers further by purchasing the DLC.
Then there's the question of sites like g2play and CJS - they'll suddenly be able to register all those cheap grey market keys on steam themselves and just sell them to you via steam, that'll kill the second-hand market for ordinary users, and I can't see Steam being very happy about it.
I'm pretty sure unauthorised reselling is banned as it is, these sites should be prohibited from selling any keys legally.
Thing is, Steam then have to allow you the bandwidth to download a game they haven't received any money from you for. So can they charge a handling fee on that basis, and if so will they feed any back to the developer?
I bet I reinstall my games way more often than normal Steam users, and by transferring the license, I wouldn't be able to do it any more.
Best way would be to charge the administration fee for each resale, Steam doesn't lose anything on it (they've already sold the game once, now they're making money again by transferring the game from one account to another), part of the fee should go to the publisher (extra money, minus potential lost sales), but in the end, I think it's the big publishers that will suffer the most, since their games are often overpriced and not worth half the money spent on them.
To be fair, it's all fault of the big publishers shoving us £40-45 games on release with ridiculous amounts of laughable DLC that ramps the price of "complete" games up to artificially double their cost.
PS what is quite likely, like you have said already, that the Digital Download providers will change their T&Cs and we will now be renting out games for a 10 year period, which in effect doesn't change much since we can't be sure how long some of the services will be going for anyway...