While most sellers are happy to take cheques, they leave buyers with little protection if things go wrong with the sale.
Don't worry about your Buyer Standing. If your seller is honest, and we have no reason to think he isn't, he will get your cheque, send your item and all will be well. Be sure to leave 5 Star Ratings for working with you.
If Propay (I am not familiar with Propay) could not take your payment, then the only payment you have made is the cheque. Keep an eye on your bank account or credit card, depending on which you backed the Propay account from. There should not be any payment processed. If there is, your credit card company will process a chargeback. You call the 1-800 customer service number.
Actually, how does one use Propay? Do you have to open an account with them? How long does it take for them to verify the account? That may have been the basic problem-- the payment could not be processed until they had verified you as the account holder.