Phone your bank and get them to put a stop on your card NOW.
Ignoring than the fact the url you have pasted is "Qumtree", there shouldn't be any reason for you to enter your card details if you are the seller, if you've also entered the 3 digit security code from the back of the card then they have everything they need to use it online (assuming they have your name & address/postcode from "arranging collection"?)
Started a scan just in case any virus/malware/spyware etc infected iMac.
Bank card stopped just in case, but no I hadn’t inputted my cards details or anything it’s just how the screen was worded.