This is going to vary from state to state. Potentially you could be considered a money services business, a money transmitter, or you may need to become licensed as an escrow service. You'll want to look into Fincen's guidance on whether you are a money transmitter. You'll also want to look into your state as well as other states money transmission laws. Typically speaking as soon as your holding funds for others or transmitting money you'll need some type of registration or licensing. MSB licensing can be incredibly expensive and a very long process. If there's any way to avoid holding funds I would do it for simplicity as well as to avoid liability.