For books, at least, it's helpful (article is a few years old) and I imagine it's the same for other industries:
| Used books, the economists found, are not strong substitutes for new books. An increase of 10 percent in new book prices would raise used sales by less than 1 percent. In economics jargon, the cross-price elasticity of demand is small.|
One plausible explanation of this finding is that there are two distinct types of buyers: some purchase only new books, while others are quite happy to buy used books. As a result, the used market does not have a big impact in terms of lost sales in the new market.
Moreover, the presence of lower-priced books on the Amazon Web site, Mr. Bezos has noted, may lead customers to "visit our site more frequently, which in turn leads to higher sales of new books." The data appear to support Mr. Bezos on this point.
There are a few other articles out there that say the same. As others here have said, it helps with exposure. If I'm on the fence about a certain game or movie or book, I can get it for cheaper and then if I really enjoy it, I'm more likely to buy other installments, especially new (so I can get them sooner). It also lowers risk by allowing customers to recoup some of their costs. It's one of the reasons I don't mind buying things digitally: I may not be able to resell them, but I'm only spending $10-15 tops as opposed to $50-60.
Same with cars: I doubt many people would buy new if they couldn't trade in when the time came.
This is why it's weird to me to see, say, video game companies rail so hard against used sales. They're more concerned with more sales RIGHT NOW and making sure if they aren't profiting off of something, then no one should, but it's all incredibly short-sighted.