Armoires have been used instead of framed closets in the past to skirt the closet requirement, it is not exactly the correct way to get things done, but it does seem feasible if there were absolutely no other option.
Conforming vs. non-conforming bedrooms are determined by the window’s size and accessibility to the outside in case of a fire. i.e. an egress window. I believe the windows need to be 3’x 4″ feet minimum, but I would check with your local building codes. Basically, the windows in the basements before 1980’s were all little 18″ x 24″ windows. These small windows were often 6 feet off the basement floor and would be really hard to crawl through in case of a fire. Egress windows can be installed into basement foundations usually for $750-1500, although I have seen quotes as high as $2500.
As for resale and the basement bedroom, it can be tricky. There is no guarantee you’re going to be able to get a huge return on that “extra” bedroom, especially if it’s not an aesthetically pleasing space to begin with. In a basement that is livable with high ceilings, a nice bathroom, and a separate laundry/storage/mechanical area a nice-sized bedroom that is conforming will add to the value of the home.
Many nonconforming basement bedrooms are used for other purposes anyway, like sewing rooms, gyms, offices, storage, etc. so it may be unnecessary to invest in an egress window in a rarely occupied space.
The bottom line is, you should have your agent disclose that the bedrooms are non-conforming bedrooms as far as current building codes are concerned.