Dealer or merchant (6)
I believe the answer is:
trader
I believe this is a double definition.
'dealer' is the first definition.
(I know that dealer can be written as Trader)
'merchant' is the second definition.
(I know that Trader is a type of merchant)
(Other definitions for trader that I've seen before include "One buys and sells" , "Dealer, monger" , "Tarred the dealer badly" , "Person in the business of buying and selling" , "Red tar (anag)" .)