it totally depends on what your site is built in? Let us know

Have you had a look into Intercom, Drift or Help Scout Beacons? I hope these help.

TalkJS is a good option and can be integrated into any site that requires user-to-user chat. However, being a SaaS product it can get expensive.

Another option that I recently used is CometChat. This is a PHP-based self-hosted chat engine available for exactly the same purpose (i.e., user-to-user chat) so you can set it up for chat between buyers and sellers. CometChat has a plugin for easy integration between the chat engine and your website. However, for deeper integration, you will need a developer to be able to hook it with your user system. Since it is self-hosted, you purchase it with a yearly-license which significantly reduces the cost.

I've successfully helped over 150 entrepreneurs, startups, and businesses, and I would be happy to help you. Please send me more information before scheduling a call - so I can give you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/ripul.chhabra