As I have already right-sized my collection (and have very little left to sell), the used train market prices don't effect my new train purchases.
A couple of years ago, I decided that I wanted to have more prototypical trains. So, I sold a bunch of stuff so that I could buy some newer offerings like 3rd Rail's Columbian and Train or Tomorrow sets.
Now, I'm pretty selective on my new purchases, and while I buy them new, have no plans to ever sell them.
Jim