If it is on Ebay, the system generates offers the seller is uninvolved with. If you are watching specific listing, the ebay algorithm tries to entice you by giving you an offer in attempt to make you bid. If you take the offer, the seller then has to also accept it for the sale to be made. I have taken offers only to have the seller counter offer.

If you are watching auctions on other sites, perhaps they have the same system in place.