How the contact button works, what the seller sees, and what Chux never asks you to do.
A listing shows two ways through: write to the seller, or show the phone number. The number is hidden until you ask for it, and asking is recorded, so a seller can see that somebody wanted their number without learning who until a message arrives.
Writing opens a conversation attached to that listing. It stays in your account under Messages, on both sides, and it survives the listing being sold or removed.
Starting conversations is budgeted: twenty in reasonable succession, then a slow refill. It exists to stop one script writing to a thousand sellers, and a person shopping normally will never meet it. If you do, the answer is to wait rather than to retry.