When i was shopping for my 55UT50 last December I had to cross state lines to find a store that carried the 55" model, and they only had 1. I find it very rare that a Sears has a 55" plasma in stock at the store due to low demand. Most people who buy plasmas are either looking for the cheap 42" model, or a maximum 50.
$900 sounds about right. It's probably about an $1100-$1200 TV brand new right now considering it's last year's model. It's likely a display model as well that has heavy use for a year. 15-20% off is fitting.