My suggestion is to buy one new from a place that offers return privileges.
I don't actually have a diagnosed back condition, but I do have a certain amount of trouble with my back from time to time. It doesn't happen to me often, but if I sit in EXACTLY the wrong position on EXACTLY the wrong kind of chair, it can make my back hurt for weeks.
So, I WAS a little nervous about buying a bent.
For whatever it's worth, I bought the entry-level EasyRacers EZ-1SC, just over $500 new. I bought it from Harbor Cycles in Hingham, MA. (My ONLY connection with them is as a satisfied customer). I raised the question with the pleasant salesperson, and he said immediately that all their bikes had 30-day return privileges. I pressed him a bit on this, saying that I had had trouble with my back and saying, "I just want to be sure, because I MAY have to return it." And I asked him to write "30-day return privileges" on the sales slip and initial it. He didn't hesitate or so much as blink.
I don't know how many bike shops will do this for you, but I suspect many will. As it happens, I haven't had any problems, so I also can't say what REALLY would have happened if I'd brought it back after a month and tried to return it.
As I said, I haven't had any problems. Well, let me qualify that a bit. It's never good to sit in one position without moving for a long time. While you're actually riding a bent, you ARE pretty much sitting in one position. I'm a pretty casual cyclist; the longest ride I've taken so far was about an hour and a half, with breaks every half-hour or so. When I've been riding a half-hour and I get out and stretch, I have the feeling that my back is saying to me, "yes, it really WAS a good idea to take a break every half hour." But, so far, there hasn't been any problems or pain, neither right after the ride nor the next day.
I probably need to be careful to scrooge around a bit in the seat WHILE I'm riding.
He said: "This front wheel wobbles." I said: "It doesn't if you don't wobble it." --Jerome K. Jerome