I think this is a silly post, but I'm going to try to be helpful.
First, you haven't told us what you even have. Secondly, are you looking for used, open, misb, with instructions, etc?
Also, bricklink has its own ratings system, so you can gauge who to buy from, based on that. We also have a reputable dealers thread:
viewtopic.php?f=8&t=14641
And a C-C member's store thread:
viewtopic.php?f=8&t=7698
Also check out the set review threads, I think that almost every castle set has been reviewed. Check the lugnet and brickset reviews for more information. Eurobricks also does reviews of vintage sets.
Steve