You are totally right Pietro. Sellers can in fact choose which PM response to their ad that they accept. That's in the rules.
So as long as the items are listed in the classy's and the buyer send a PM with intent to buy the sale meets the rules.
Posting Etiquette
6. First come, first served basis doesn't mean seller must sell to the first member to contact them. It is the seller's prerogative to sell to whom they feel most secure selling to.
ECF Forum Rules
15. Advertising/Selling in posts
a.You must not use this forum for advertising. Any post that seems to have been made for advertising will be deleted. Registered Suppliers may advertise subject to compliance with their specific rules. Members wanting to sell, trade and/or buy e-cigs and e-cig related items from other members may only post these items in the Classified section.
b.Shilling (excessive promotion) is not permitted. Includes affiliate codes/links, links to Unregistered or Banned Suppliers, and links to sites of dubious quality/content/reputation in signatures, blogs or forum posts.