What is preventing you from buying this item? What is the message you are receiving?
Have you tried switching devices and browsers? If using the app ( very temperamental, and some items cannot be purchased through the app) try a laptop / PC , and try Chrome that seems to work best with eBay.
I'm presuming you do not have any unpaid item strikes ? These last 12 months and can prevent you buying from most sellers.
@leew7573