Starting small and evolving your business based on actual demands (purchases and inquiries) is an excellent idea. If you have the budget and time, attending Taipei Cycle will give you direct contacts with Asian manufacturers, many of them produce mini velo and folding bike parts that are hard or impossible to find here. E.g., carbon forks for small wheels.

Folding stems are really hard to find. In most cases you have to buy a bike to get one. It should be just a folding stem, not with the handlebar built in, so you can set it up to fold right or left, front or back.

Long seat posts are very hard to find. You'd want to stock two or three different sizes.

With rim availability, the issue is usually the spoke count. 36h rims are not too hard to find, but 28h can be a real challenge.

long seatposts are hard to find ? Well.......... we stock 9 different ones
http://www.thorusa.com/accessories/seatpost.htm
working on a very special one on top of this, which would make the seatpost count 10 ....:-)

I assume you are looking toward an online store. Finding the right balance of immediate inventory of the most popular parts, and access and even special searches for specialty parts that are not inventoried will be the key to you success. I got my first folder less than a year ago. Just to get the seat post assembly and cable routing set up to work for me required ordering from New York and Hong Kong folder stores, a BMX shop and a unicycle shop, and my local ACE Hardware. One stop shopping would be great.

all thoes things that bike shop sometimes dont get
like extra long cables,telfon linned, kool stops or other soft pads pads
handle bar extentors,
dahon adres stem sateri hallo
Qr peddles
luggage,
Mezzo parts online