Well, the answer is yes and no. Reaction was built to support multiple shops from the beginning. The schema supports tying almost anything to particular shop.
What that means is that you don’t need to get way deep inside the internals of Reaction to make it work.
We have not really made a lot of changes to the front-end that you one might need to make it work in a “marketplace” scenario. And part of the challenge there is that there are many, many ways that this might get implemented (is this an Etsy-style marketplace or an Amazon-style marketplace, or something else?). So it’s difficult to make this something where it would fit every use case.
So if your question, “will Reaction work Out-Of-The-Box for a marketplace?” I think the answer is no. But if you watch what other people are doing people are building marketplaces on Reaction so it can be done (and I think Reaction is a good platform for doing it)