As long as your shipping costs are stated in the listing, an offer is supposed to be for the item price only, according to eBay policy. The wording on the eBay buyer "help" page dealing with Best Offer in this regard is right at the bottom of the page:
For listings in which the seller has specified shipping costs for the item, the Best Offer price includes only the listed item. For listings in which the shipping costs aren't specified, the buyer can choose to include shipping costs in their offer.
Strictly speaking, it's up to the seller to specify in the listing whether shipping is included, not the buyer when making an offer on a listing with shipping shown. However, that said, I have had a couple of buyers who either misunderstood the "Best Offer" system (or, more likely, deliberately wanted to circumvent the policy) and made an offer on the terms that their offer included the shipping.
This creates a problem for the seller, especially if it's an item you'd like to sell quickly but which has a fairly high shipping cost. What do you do? Refuse the offer (which you're completely entitled to do), or swallow the bitter pill and make the sale? Or perhaps make a counteroffer and try to explain to the buyer that his/her offer is actually not valid, risking upsettting the buyer.
Such an offer is obviously much lower than the stated amount, because you as seller will be paying for the shipping at your end, i.e. giving the buyer free shipping. In my two particular situations (both of which were UK buyers), I decided to accept the offer and absorb the shipping because the value of the item was high enough, but I did send a polite message saying that I'd pay the shipping this time, but that a buyer's Best Offer is actually intended to be for the item price only. Frankly, I think both buyers knew very well what they were doing, as they were both experienced eBayers. It was a little bit of a scam, but the offer in each case was just enough to make it worthwhile. They both got free shipping via the back door, but I did make two good sales - c'est la vie.
However, I've used Best Offer for years, and these were the only 2 instances where a buyer attempted to "work around" the Best Offer system to get free shipping. I don't think it was an accident that they both happened to be in the UK -- with the outrageously high customs charges on incoming purchases, UK buyers may be more tempted to ignore the rules on offers. And of course, there are always the occasional opportunistic "lowball" offerors who can be kind of fun to play with in counteroffers. Generally speaking, I've had very good results with Best Offer.
