Fred,
First, welcome to the association and the forum! I came back to muzzleloading a few months ago after about a 10 year hiatus and decided to try something new, a smoothbore. I've always been an accuracy nut and so always shot rifles. I bought a .62 cal (20 gauge) French fusil fin made in 1990 by Centermark Arms off a guy on Gunbroker for $850 plus shipping and I spent another $70 for a ton of flints and other stuff he had for it. That smoothie has been an eye-opener. It may lack the long range accuracy of a rifle but at 50 yards it creates a one-hole group when I do my part. I'm looking forward to trying it with shot. "Fusil fin" means "fine gun" and this one has a curly maple stock that will knock your eyes out. You can probably find one with a plainer stock for less than I spent but none of the smoothbores seem to be exactly cheap. Since you specified "used" the best I can suggest is to hit the forums and online gun seller sites and keep looking until you find what you are looking for. Good Luck! I can tell you that while you may not "need" a smoothie, once you get one you may be selling that SxS.
As for what to look for, I'd suggest the same things that you would look for in a rifle. A high-quality lock is a must if you are going to avoid frustration. A good barrel is worth the money too. There are a lot of made-in-India guns out there right now for cheap prices. Many are okay but be aware of the fact that some are very cheaply made and don't even have a threaded breechplug. The crappy ones have a steel plate that is welded over the breech end of the barrel and there has been at least one documented case of the the plate coming loose and rupturing. Be sure you know what you are getting. The only difference between a pipe bomb and a gun is craftmanship. We have some people on this forum who would build you a quality smoothie at a reasonable price and you wouldn't have to worry it blowing up. I owned one of the India guns in percussion and sent it back after I discovered a serious casting flaw in the patent breech.
John