I recommend some homework. Create a part file that is similar to what you want to machine. It doesn't have to be the most complicated part you'll ever do. Just the basics, perhaps a pocket of some sort and possibly a profile to cut the piece free from the balance of the material. Bring your own material (plus extra) and ask to run the machine. Get your self an appropriate end mill and try the thing out. If the seller is at all motivated, he won't have a problem with this.
I own a 1997 XMTC 17x20. It provided a good education for the Tormach I purchased recently, but I was never satisfied with the milling ability of the 17x20. Cuts were never what I would call clean. Even in plastic, tooling marks were uneven and very visible. The 3 axis ball screw upgrade did not help this situation to my satisfaction. The Bridgemill feature may improve that situation and you may be a more skilled (and patient) operator than I was with the 17x20. That said, I produced functional parts with the 17x20 in both manual and CNC mode. I keep it around and have converted it to a CNC lathe. I find it acceptable for this task. The nice thing about owning the 17x20 is that it truly makes me appreciate the capabilities of my Tormach.
As to the price... I wouldn't pay more than 1/2 the new cost if it is in good shape. Please remember the tooling is not an insignificant adder and can easily be over $1000.00 in itself. I know nothing of the going prices for a Bridgemill. I am offering this based upon my satisfaction with a similar product. It is only an opinion. Perhaps, some other members will chime in and contribute their 2 cents also.