When I sell/buy used, I tend to use glasscages.com as a baseline.
For example; new 125 wide = $350 or new 125 tall = $295.
Even for those not in GC's truck delivery range, their prices tend to be relatively close to what I can get AGA pre-ordered from any LFS (that's an AGA distributor). So unless you have some non-standard (or high-demand) size, I would assume people can buy new relatively close to glasscages prices.
So then I adjust accordingly for whatever the used condition is and what I expect people to pay off retail. Personally I don't look at used items unless I'm saving 25% or more, just isn't worth it in my opinion. Spending an extra 25% for new silicone, clear glass, and warranty, is worth it to me.
Ed