Offering to buy via PM is permitted but only on a thread in the appropriate "Sale" section. The sales section rules require a price to be stated.
Offers to buy (however made) on the back of a valuation thread are not permitted because that would then circumvent the rules in the sales sections, turning this into an auction site and removing the need for a "sales area".
The current approach is open and fair IMO, whilst being reasonably easy to administer.
Should this be in Non Airgun Related?