Yep, the seller has , and perhaps inadvertently, set their site preferences to block buyers buying multiple items in a short period of time.
This is an option, as it can stop malicious buying , buying many items over a few days, opening cases and leaving negative feedback, This option selected prevents this happening.
However, it's an easy thing to mark by mistake, too.
Send your seller a message, ask them to check their site preferences and place you on their exemption list, that is, if they do wish you to buy from them again ! 🤔
@sur-plus