Where are these marketplaces?
They're actually extremely easy to find. Just do a web search, and you'll find countless data exchanges where you can purchase personal data.
Many of these are massive, publicly traded companies.
Who can buy your data?
This leads to another question: Who can sell your data?
Or an even larger one: Who really owns your data?
The issue is none of this is clearly defined legally. You voluntarily give much of your personal information to sites you use, expecting it to be used for what it was intended. Aggregators legally acquire data through means opened by sites you use.
Technically speaking, sellers do legally own the data they sell, even if it's your personal information.
To answer the original question, anyone can buy personal data. There are no regulations here. You could go on a data exchange and buy your own data to review. In fact, we encourage you to at least once, as it may inspire you to be more careful online.