Niches are specialty areas that one focuses on, in addition to your regular bread and butter business.
Some framers mix gift type items with the custom framing operation.
Most of these type niches require an inventory. Posters can be ordered through catalogs. So this too requires an inventory. You could combine inventory with supplemental catalogs. In addition to specializing in pre-finished moulding my favorite niche is very profitable but requires equipment, space, inventory and manpower.