Bit late on this post, but I hope this helps a bit. I totally agree with the posts here with book values, only worth what someone wants to pay.
The Unseen Library is a strange beast indeed. A desirable book club edition, which breaks all known rules of book collecting. These are usually sold individually, half set (9) or full set (18), plus the most collectable (IMO) Maskarade (the mispelt printing).
Normally the earlier the book in the authors repetoire the least amount of copies printed (they aren't famous and collectable). As they gain fame and popularity the amount of 1st print runs are increased to meet demand. However with the Unseen Library editions it is turned on its head. More people ordered "The Colour of Magic" 7000 copies printed, than the final book printed "Maskerade" 3000 copies. Therefore rarity should make Maskerade more collectable and thus more expensive........ wrong. The more common early novels fetch the most money....... I guess it a perceived rarity based on the original book.
Selling prices... Do not believe what people attempt to sell them for on Ebay, most are dealing in them and try to inflate the value. I always look at dealers on Abe books and divide by 3.... yep three. This is because that is their livelihood and they have to pay business rent etc etc. They also need make a fair margin with scope for bartering. Many on Ebay set strange values. Have a look at what they have sold for in the past and see what opening offer was.
To sell pick a fair price to offer up at and then you will get more interest. If you put "The Colour of Magic" out starting at £100, you will probably just be giving Ebay free money for insertion fees, as people will more often as not, ignore it as greedy. Always have a price in you head that you will be willing to part with them at.
I have seen Equal Rites for anywhere between £80-£120. I came across one cheap £4.00, decided I didn't want to collect them and sold it for about £40.00. I made a good profit against my purchase price and the buyer got a good deal based on the normally advertised prices. All good all round
Good luck if you are selling them. Sorry for meandering along