You should buy it if you like the sound of it and can afford it.
As for what it's worth. Try doing a search for completed auctions on Ebay for drums of the same spec and similar drums to get a ballpark figure. Auction results that come up green were items that sold and should give you an idea of what the drum goes for. Results that come up red did not sell and can give you an indication of what is too much to ask.
Also, ask these questions:
1. What does it go for new? Remember to find out what it actually sells for in big shops, not the recommended retail price.
2. How much did he pay for it (if he'll tell you) and how much is he selling it for?
3. How old is it, what kind of use has it had, what condition is it in?