Common types of unwelcome bidding include:
Bids by prospective buyers who live in areas outside the auction's stated shipping area
Bids by prospective buyers with low feedback scores on auctions whose terms include a minimum feedback score for bidders
Bids by prospective buyers who have more negative feedback than auction terms allow for bidders
Though bidders who are warned for unwelcome bidding are often frustrated by restrictions that exclude them, logic also suggests that it is not in their best interest to attempt to do business with a seller that does not wish to transact with them. Unwelcome bidding is thus a bad idea on all fronts.