if you can get materials for cheap and there is enough local demand your golden.
you might wanna get involved with a local horticultural club or reptiles club, basically expand your market.
you can make fliers and put them up in markets, conventions and public place.
its perfectly legal just stick to that dont grow or hold large personal stash cuz if you start getting popular you want to be clean if the fuzz starts sniffing.
spoke to a guy on craigslist he been selling grow boxes whole systems and partial systems for 4 years. seems it involves alot of marketing to right people. alway keeping eyes peeled for deals on materials. most important to stand out from the others try to use imaginative stealth boxes like mini fridge and and nightstands and even aquarium stands.
if you are already building them then you have to explore the possibilities.
keep prices as low as possible and use creative marketing once your established and create a buzz in your area you can start raising prices. but in the beginning its gonna be more for the love of it keep that in mind and you might have a good bussiness plan.
goodluck
Philo
(this is my first post by the way!)