First you'll need the 'sellers'.
To get them to sign up you'll first need a website set up that looks professionally made to the extent that one could conceivably believe that it would attract 'buyers' to use its services.
Then, do research on how best to find sellers that would be appropriate for your site. You'll have to do grunt work to reach out to them individually at first. For instance maybe you're looking for experts on certain topics, you could check out slideshare.net and find the most popular decks on relevant topics, and then reach out to those authors. Or maybe you check out Linkedin or Quora and find the right people there. Help them with the sign up process as much as possible, and give them some extra early-sign up incentive, like maybe say that you'll feature all early sign ups at the top of the website for 1 week after you go live, or maybe you'll actively email market for them for several days each.
Once you've gotten sellers to populate the site, start advertising to buyers. Give the sellers easy ways to do advertisement too. For instance give them text and/or photos for social media posts, etc. If you onboarded some already-popular people as sellers, they will be able to help a lot in terms of generating their own traffic.
Although eventually P2P marketplaces can be somewhat self-perpetuating, initially you'll have to do a lot of manual work behind the scenes to get everything going.
best of luck,
This almost always effectively comes down to contact database abuse.
Linkedin Grew and got a 10M USD fine after importing addressbooks of users and inviting a ton of people to the platform.
Ebay and Paypal had interplay requirements.
Airbnb abused craigslist bots in the early days.
There's always platform abuse to grow a P2P marketplace.