The description isn't clear, but you have to sell 1000+ goods in a single game loop of 60 days. This'll be pretty tricky unless you buy a vending machine or two (They hold 10 items each and are sold in the Guild) and stock them up with items bought from the Guild for resale.
Vending machines sell items for a flat 100% of their price so the profits won't be large, but they will always sell anywhere from three to all ten of their items each time you open shop and without you having to haggle. You can still get the achievement without them, it'll just take quite a bit longer.