A bid or purchase on WARSTUFF is considered a contract and you're obligated to purchase the item. However, if you feel that you have a legitimate reason for not buying the item, you can contact the seller and explain your situation. Many sellers are willing to work with you if you communicate with them openly and honestly. Possibly, things can be worked out to everyone's satisfaction.
If the seller doesn't respond in the way you hoped, keep in mind that failing to purchase an item you have won can result in negative feedback from the seller. If you get too many negative feedbacks, your account may be restricted or suspended.