How long has he owned them ? I would ask that question to the seller & I would also ask for a sales receipt if it's available. Like anything else there is wear and tear especially as you mentioned to the ear pads & headband but more importantly you should check to make sure that both sides sound balanced & undamaged meaning any unwanted buzz or noise. When you say "if I'll check out the headphones" do you mean you live close enough to the seller so you can examine the HPs in person ?
+1 on this. Many people think the 650's clamp too tightly, so they bend the metal cup extensions outward to alter the fit. I would rather buy a pair where this had not been done (I found mine great right out of the box, and they only got more comfortable with time).
It's great you live so close to the seller. Bring the source you'd listen to at home and go try them out. Bring your current headphones too, so you can compare.
In general, I've had great experiences buying and selling goods here. There are a lot of great people on Head-fi and some awesome deals to be had on the B/S/T forums. That said, I've certainly bought new gear too (including my 650's). Either way, good luck!