Do you mean you put in an Offer for a Best Offer item and it was ignored, then disappeared? This is allowed. A seller should respond to an offer, but not all do. if the Offer is left to expire, the seller could remove the item entirely.
You could always contact the seller and ask if the item will be relisted sometime.
If the seller is unavailable, and has no other listings, they may have been removed by eBay. Or the seller may have had an emergency and pulled all listings.
If you made a bid on an Auction item and the auction time had not ended, but the item disappeared - that is an issue. And is not allowed. Once an Auction item has a bid, the auction must complete (with few exceptions).
