Marsh is a batarian merchant on Omega who owns and runs the Omega Market. He is having trouble competing with the Black Market, and is struggling to make a profit. His shop sells a variety of items, including Model Ships and weapon enhancements.
If Shepard tries to obtain a discount, at first, the Commander will walk away, saying that they will purchase items from other stores. Marsh replies saying that the only other items as good as his are in the Black Market. Shepard then turns back, and persuades Marsh to give them a discount on his items, so long as it is kept a secret.