A guy I know runs an internet powersports parts and accessories business and has been quite successful.His business model is to charge minimal markup and do very large volume.He basically undercuts the dealers price wise by going directly to the manufacturers.Manufacturers are supposed to protect their distributors who in turn protect their retail dealers but in the present difficult sales environment it's a free for all and anything goes.
A market niche I have been eyeing is the blown up 4T powersports market (I think a person would be quite busy!).If you have very good machining/welding and mechanical skills you can repair damaged engine castings and save the customer a bundle versus having to replace everything.When I had my motorsports business I had a full machine shop and did a lot of this.Examples of engines I repaired was an LT300 ATV that blew a rod and cut the cases in half and a KX250 that threw a chain and smashed up the cases.In each case I welded up the damage,remachined everything to spec and saved the customer a small fortune.It's actually quite satisfying and enjoyable work but "you gotta have a feel for it".