These are the methods you can promote your marketplace without spending a single penny. You will get signups for sure.
1. Newsletter Promotions:- Create a List and Send Simple Newsletters using MailChimp.
2. Be Active on Social Media - Facebook, Twitter, Instagram, LinkedIn, TikTok - Use Hashtags, Use Canva [Free] to Create Graphics.
3. Quora:- Find Questions and Write Answers related to your business. Add disclaimers.
4. LinkedIn:-Search and Find decision-makers of the targeted audience, Connect them with a message.
5. CraigsList: Submit 1 Post every day, explain all about your marketplace
6. Classified Ads:- Create an ad copy and post at least 50 ad posts on local classified sites.
7. Reddit: Get involved in the ongoing discussions. Share your experience and story of building the site/business.
8. SEO Things: Improve your website, at least do On-Page optimisation and target long keywords. Helps.
9. Group Marketing: Use Facebook & LinkedIn Groups - Discuss the niche problems and your services
10. Coupon/Giveaway Marketing:- Create some coupons or Giveaway offers and submit the deals on aggregator sites.