There's really no way to tell how long they will last. You could have a set of ten year-old pedals that were on a bike that somebody didn't ride 100 miles, and has been sitting in their garage for the last 10 years- or you could have a set of used pedals that someone put 20,000 miles on, and the bearings could start getting sloppy next week. I'd imagine though, with pedals, if they're in nice cosmetic shape, it would likely indicate that they didn't have a lot of use on them. Also, when buying on Ebay, there's a big difference between the guy who just lists a pair of pedals, and doesn't say much or anything about their condition, vs. someone who gives a lot of details ("Hardly any miles on them; just switched to a different kind of cleat system, so I don't use them anymore.... They're in excellent condition") and who takes a lot of pics.

Just about any name-brand quality component, no matter how used and abused, will last longer and perform better than anything on a Walmart bike, though....

I don't think I'd pay $60 for used pedals though; generally, anything used, I want to get it cheap enough to ensure that if it doesn't last too long, I can buy another, and not be into them for as much or more than new ones. Gotta be a real good deal...or I'd just buy new.

Search around the internet too- sometimes you find older new-old stock items (NOS) brand-new for less than what someone wants for old ratty ones on Ebay.