There are only a couple of rules that I can see.

One is about postage - if a seller has stated postage price separately - then that should not be included in an offer, the post specified stands as part of the offer. 

If seller does not have stated exact posts costs, it can then be included in the overall offer if buyer pops in to their offer- postage included.

The time frame for best-offers just states that offers expire in 48 hours. 

Exclusions identified in Questions and Answers on Best Offer  primarily relate to out-of-stock quantity blocking and buyer restrictions such as being on blocked list. 

Fortunately IMO this is a market place, ebay can only restrict trade and negotiation to a certain point.