Ironically, I am in the market for a new seat. Typically, if you want to sell something here on the forum, post it in the "For Sale" section. Including pictures is always a bonus as well as a good description. Include a fair price and indicate how you want to handle shipping charges. Usually FedEx or UPS are good for large items like seats.
As far as payment, PayPal works the best. You can go to their website and see how to set up an account. Once you receive payment, then you ship out the item. The burden of getting burned is always on the buyer, because if I am selling something, I am not going to ship it until I get payment.
I hope this helped explain the process.
Let me know if you have any other questions.