I think there's definitely a market out there for bespoke quirky bird boxes and bird tables (which don't fall over in the wind) I would go for the upper end of the market, aimed at those living in suburbia who want something a bit different in their garden. Everyday products too, of course, but your mark-up on individually designed things would be greater.
Insect houses are currently popular and could be as individual as your imagination would allow.
Poultry housing seems a good bet too, flat pack, or built up by you if local, for the non-DIYers. I think most people want a small house with a wire run attached, which they can move around. At the same time, there are smallholders who want larger but easy to clean houses. How about hen houses which look like little cottages, or castles?
I think that on that scale, people are proud of their hens and want their houses to be a bit of a work of art, a feature, brightly painted, to get attention.
I have no experience of running something like that as a business, but with today's internet sales and fairly cheap transport costs, mail order would seem to be the way to go - no overheads but a potentially wide client base.
I think the key is to have a good website, with all the correct key words and number of hits to keep you on the first page of any search - quite a challenge. A slow loading website just won't work. If people have to wait for photos to upload then they're gone (remember, folk in the country often have slow broadband or no broadband)
I think it's worth a try, and really you've not got much to lose with minimal set-up costs.
Do plenty of research first - what's already out there, what is most popular, price ranges etc ?