I'm with you on eBay sales. I never use the auction format, and if I every buy anything that's auctioned I snipe in the last few seconds.
However, I don't understand what you mean by your last comment, "On eBay you have zero clarity on best-offers which, in my experience, range from 100% to 25%."
I usually set the lower limit for notification of best offers at the minimum I will accept, usually 75% of my BIN price, automatically rejecting lower offers. I have seen a number of instances where the potential buyer keeps increasing his offer until he gets over the threshold. Rather than dicker I always accept that offer. If I make an offer that is explicitly rejected by the seller I will counteroffer to split the difference, but I won't deal with someone who will only go $10 discount on a $500 item, unless of course it's a must have item, in which case I would have taken his BIN price immediately.
Thanks for your comments.