Sales on this forum and all others must comply to a number of rules which can vary by forum
These rules are to help both buyers and sellers with a safe and transparent sale, e.g. A named & dated photo to proove the seller posses the items, and that the one shown is the one for sale, and doing all business in the thread so that in the event of a dispute everything is out in the open
If the seller has their own additional rules for what they do or dont want from buyers then they can be stated, eg no lowballing, postage included/to be added, no trades, no dibs etc
If they don't state such rules and someone offers then a polite no never hurts