If the seller listed the item for sale using the bog standard selling form then it may well be the case that eBay have tampered with his or her listing and added in the Best Offer option without the seller's consent, hence the reason why the offers are being refused. Another reason as to why offers often get refused is when buyers make an offer that is so low it's practically insulting. I've had people do that to me in the past when I used to sell on the site - if people sent me insultingly low offers I'd put them straight onto my Blocked Bidders List.
Although eBay originally only offered the Best Offer option for Buy It Now listings they started to make it available for Auction listings a few years back. Whilst this can have advantages for the seller if somebody makes an acceptable Best Offer before anybody has placed a bid for the item, if a buyer makes an offer that is actually lower than the starting price then the most likely outcome is that their offer will be rejected. If you see the Best Offer function on an Auction listing then don't make the mistake of thinking that the seller is open to offers below the starting bid - the starting bid already shows you the minimum amount that the seller wants for the item, so if you must make an offer then make the seller an offer that's greater than the starting price, and submit a decent offer that you feel may possibly tempt the seller to accept the offer and sell the item to you. If you make a high enough offer then the seller may accept it, in which case you won't have to wait for the auction to run its course; however, if the seller rejects the offer then you'll either have to place a bid for the item (in which case the Best Offer function will vanish from the listing and cannot be reinstated) or see if you can find a similar item being advertised for sale by another seller who's not asking for as much money for the item.
