Have you sent these sellers a message asking why you are blocked, could they cancel the block to allow you to buy. They of course do not have to oblige, but worth asking.
A few reasons why you may be blocked:
Do you have any unpaid item cases raised agianst you? This then will block you from the majority of sellers for 12 months.
Or, buying mulitlples of items from one seller, that seller may have in their site preferences to block anyone buying more than a certian number within a short period of time. Some sellers may not even know they have selected this option, so worth sending a message to ask.
Finally, if you are trying to make or accept an offer, eBay now require an immediate payment to be made, so a payment method needs to be registered for this.
@2222mag