how many rounds is a barrel good for?  I know a guy who want's to sell me a Bushmaster AR upper for $300.  He says he thinks he's shot 1500 rounds through it.  I thought that sounded pretty worn out.

I have a Rock River with over 4,000 through it and still running strong.

Worn out??  It is just barely broken in.  

Just inspect the bolt and bolt carrier parts, look through the barrel, check the gas key/gas tube fit for interference (by sliding the carrier into the upper without the bolt, should do without binding), key tightness, etc.  Should be good to go if it is what he says.  

If it's been well maintained, 1500 shouldn't be a problem at all, especially on a chrome-lined barrel.  I would get the headspace checked to your bolt, otherwise I wouldn't worry about it.  I believe I read once that the chrome-lined barrels are supposed to last around 15,000 rounds, but again, it depends on how well it's been taken care of.

It all depends on what your standard of expected accuracy is.

1500 rounds through a properly maintained AR is nothing.  This is not a "tolerances so close it's barely supposed to work" rifle.  It's a military-intended workhorse, and it WILL function well even after being overworked and abused (sort of like GIs).  But after several thousand rounds, an unlined barrel may start to show slight variations in point of impact, especially after a lot of hot rounds.  A chrome lined bore should stand up to a lot more use before you see any loss of accuracy.