yes there are several AFAIK. Who comes to my mind is scratchbuiltmike - Michael Stamey/ South East Scale Models. He has an ebay account where he frequently sells cars. Years ago I purchased two of them and I am pleased with the quality. Here is a (used) example:
Gesendet: Montag, 03. April 2017 um 19:54 Uhr
Von: "Jared Harper harperandbrown@... [STMFC]"
An: Proto-Layouts@..., STMFC@...
Betreff: [STMFC] Resin car assembler wanted
Apparently unlike many model railroaders who do not plan to finish their model railroads I would like to get mine mostly complete before I croak. I really love the prototype research and I like operation, but all the stuff in between like building models is not a big thrill although I am getting structures and scenery built. Anyway I am looking for someone who assembles resin kits. I have been sending kits to Pierre Oliver who does a really outstanding job but isn't getting them built as fast as I would like. Is there someone else who assembles resin kits for pay?