I'm having similar problems, and although I sell a stack of prints on eBay of my Superbike and MotoGP art. Getting into somewhere like GrandPrix Legends for example, to sell your work is darn near impossible.
I have considered setting up my own dedicated moto art website and inviting other like minded people to join in on the venture... The problem is the advertising side of things, actually getting people to come by your shop!
Keep plugging away, you'll get there
All the Best,