Good luck to you...I am in Mass and have been looking for a used one too without much luck. Most of the ones I have found were huge (commercial size). I was hoping to have one in before winter, but I may need to add it to my build-it-myself list for next year.... [img]/forums/images/icons/smile.gif[/img]
I don't know of any used ones, but have you considered building a small hoop house. A plastic covered hoophouse is easy to construct and a lot cheaper than a glass covered house. There is a company in MA that sells kits at a reasonable price. I looked at them a few years ago and they were about $400 - $600 for the size you're looking for.
A number of auctions have occured in the NE area selling the hoops for a 30x60 or whatever size you want a greenhouse for $300. Now you need to add the new poly underground water etc. There are deals to be found.
For a 10x 12 tho I would build one with used glass doors and a timber frame structure. You can get used glass doors or patio sliders from a junkyard and make them into a really nice structure if so inclined.