The seller has inadvertently set this to happen, in their Site Preferences. it is an option for sellers to select if they wish, as it can stop a disgruntled member buying a multitude of items in a short period of time, and then trashing the seller's account with negatives and bad feedback.
Advise your seller to go to their Site Preferences >> Buyer Requirements. if they scroll down that page, they will see that they have ticked and selected not to sell ** items to one buyer in a period of ** days. They need to untick that box, and submit changes to current listings.
That then will allow you to buy more than one item from them.
