I think that's the unfortunate thing about selling textbooks back. Unless you do it soon after you've bought them, it's likely a new edition has come out, making yours almost worthless.

I still have many of my undergraduate textbooks. They've come in handy in graduate school, but someday I'm probably going to come to the same conclusion that they're just taking up space. Maybe I'll find someone who can make use of them.

I suppose the content of mine won't be irrelevant, since most of the basic principles of chemical engineering haven't changed and won't be changing any time soon.

Anyhow, glad you got something rather than nothing!