I think they block phones if you ask, because it was stolen by a genuine thief. Once a contract is signed, the singner is the owner of the phone, whether they subsequently pay or not.
But in any case, buy from a reputable ebay store.
Definitely an area to be careful.
I recently delt with this mob on ebay. They sell refurbished. You select condition, colour & memory and get a price.
I used the 14 day change of mind policy and it worked without issue (sent it back and got a refund).
Seems stock dependent so currently just one combo, but presumably changes fairly fast.