Playing devils advocate, the 'What's your best price?' could be seen as a way of cutting through unnecessary too-ing and frow-ing (sp?), and getting down to business.
In it's simplest form, it is just a different way of getting to an agreed price.. so say the watch is $3000. Buyer asks for the best price, and the seller responds with $2800 - then the buyer agrees at $2800 = nice and simple.. but.. the seller may be thinking that their actual best price is $2700, but says $2800 in anticipation of the buyer offering below the 'best price'.. so strangely, this seemingly more direct & business-like system may not even benefit the buyer.
But still.. in my old-school mind, it's incumbent on the buyer to make the offer, not the other way around.

Personally I don't think that there needs to be a set of rules for selling a watch, as I like the higgledy-piggledy of individual sales ads. I guess what I'm saying is this - it's not the way a watch is advertised, it's more about the way a deal is done. I personally don't have a problem with people inviting offers.

Stewart
Heuerville