Refund Guy is a human who repeatedly attempts to obtain a refund for a product he bought from a Turian merchant within the Markets district of the Wards, one of the many sections of the Citadel. He has failed with every one of his attempts to this very day, due to the lack of a proof of purchase.
Refund Guy makes a return in Mass Effect 2 where he can be seen talking to a salarian near the lower Zakera Wards warehouse. (He still hasn't got his refund.)
Refund Guy makes another return in Mass Effect 3 where he can be seen talking to the Aegohr Munitions salarian sales clerk. If Shepard supports Refund Guy, the clerk will return a full refund--all fifteen credits of it. If Shepard supports the sales clerk, Refund Guy will realize that "the middle of a war" is a bad time to demand a refund, and the sales clerk will admit he's never seen anyone so dedicated over a toaster.