Selling parked domains with parking services

Hello. If I park a domain on a service like Sedo, and if a buyer makes an offer and pays for the domain, am I obligated to sell it at that price? Or can I refuse the offer? Because I read about a user who received an offer, the buyer confirmed/paid for it and then had to sell the domain, without being able to refuse the offer. Or does it depend on the parking service?

Selling with BUY IY NOW option means you are obligated to sell.
Selling with MAKE AN OFFER and related MINIMUM PRICE means you can make a negotiation.