Any concern over the "used" status of these panels? Do panels degrade over time?
Assuming these are true TM50 and TM55 panels, no, I wouldn't worry. Monocrystalline and Polycrystalline solar cells, like those used to build the TM50 & 55, degrade slowly, loosing up to 20% of their rated capacity over the course of 30 years, so after 30 years of use a 50 watt panel will still pump out 40 watts. A similarly aged amorphous panels will produce less than half their original output. (Panels constructed of crystalline solar cells have a "postage stamp" appearance where cells are arranged in rows and wired together; amorphous cells typically have a single surface that looks like a solid coat of dull black paint
What you do need to look out for in used crystalline panels is damage due to storm damage, hail, and other impacts that can crack the cells. Take a good look at the pictures of the cells you make offers on and make sure any panel you buy on eBay comes from a guy who has good feedback on lots of sales and check his or her most recent feedback to make sure s/he hasn't gone downhill on his/her most recent sales. Only buy panels that the vendor claims to have tested and guarantees will work, then, when you get your cell, take it out and point it at the mid-day son and measure
1) its output voltage, which should be comfortably over 17 volts and
2) the amperage the panel delivers to a short circuit.
[b]Make sure you know how to use your multimeter and that it is rated for 10 amps, otherwise you will blow its internal fuse or fry the meter.
The panel's actual maximum output will be about 75-90% of the measured short-circuit amperage, and the output in watts is this reduced number of amps times your charging voltage, typically 14.5 volts. For a 50 watt panel you'll also need a solar panel
controller or you'll boil your battery
Is this the same guy you got yours from?
No. I have no experience with this particular seller, but I'm feeling very tempted . . .