What's the difference between me playing a used copy of a game and an illegitimate copy of said game?
If your goal is the developer seeing revenue, they wont on either side.
#For argument's sake
John Doe might buy a new game for $60, knowing he can sell it used in a week for $50 (after he beats it). Developer gets $60.
If John Doe believes that there is no market for used games because everyone who might buy used will instead get a bootleg. He waits until the prices for the new game is $20. Developer gets $20.
(I'm assuming the developer gets everything, but obviously that's not quite correct - publishers and retailers get their pieces of the pie)
To go a little further off-topic, this is why it is so short sighted to complain about used games sales. Here's my standard game purchase history if I really want a new game: Preorder (Developer/Publisher/Retailer get $40-60); Beat it as quickly as possible, then sell it used; Buy it again later (new or used, depending on prices and perks) to keep in my collection.
If I can't sell used anymore, I'll definitely take that into account and never preorder or buy at more than $20/ game again.