I'm looking for a way to describe a small shop that sells single items to few customers, such as pieces of art, antiques, etc. Is there any umbrella term for this kind of business?
I'd suggest boutique
1.a. A small retail shop that specializes in gifts, fashionable clothes, accessories, or food, for example.
1.b. A small shop located within a large department store or supermarket.
2 A small business offering specialized products and services: an investment boutique; a health-care boutique.
To me, a boutique is a shop that specializes selling women's fashions and accessories.
Instead, a shop that sells a little bit of everything; vintage clothes, ornaments, curiosities etc.
I would call a bric-à-brac
Bric-à-brac or bric-a-brac (origin French), first used in the Victorian era, refers to lesser objets d'art forming collections of curios, such as elaborately decorated teacups and small vases, compositions of feathers or wax flowers under glass domes, decorated eggshells, porcelain figurines, painted miniatures or photographs in stand-up frames, and so on.
I think emporium might cover it in a sort of ironic way.