I agree with digital*ghost, Best offer is about the price, any other stipulations should be openly negotiated with the seller. For example, I sell sewing patterns and quite frequently I get notes from the buyer in the payment stating that they want it in a certain size. After talking with the buyer, they want me to actually make them a dress. (the mind boggles as to why someone thinks they can get a made to measure dress for $10 with $1.50 shipping). At no point in my listing does it say it's for a garment, it clearly states it's for a sewing pattern so you can make the items yourself, and it's listed under the correct category but I still get people frequently trying to get me to do it and none of them actually message me about it, they just buy it and then give their conditions.

I realise this is an extreme example and differs from the best offer issue, but by saying conditions of the sale can be changed with a best offer, it can really get out of hand and unlike terms and conditions set out by Ebay, buyers tend to not be aware of legal phrasing and can quite often be unclear about their requirements (especially those who know they are being downright cheeky).