If they are version 3 license types, and come with the addons, then around $100 is reasonable if you ask me. Especially if there is still time on the license. Since it allows users to choose between version 3 and version 4.
If they are expired, and without addons, then I think between 50 and 100 is reasonable.
The market of course changes the value a bit. I think it is what people want to pay for it, and I hope that some people are still interested, and bid up or place an offer. Good luck!
I have no problem with people putting their license up for sale, be it xenforo, vbulletin or ipb.