Without commenting on the suitability of each and every library option, I will say that BitcoinJS can do what you need here. They provide a set of library functions that can create multisig addresses with any set of public keys and M-of-N requirements as well as signing functionality to sign those with private keys (which your buyer and seller could do on a browser hosted solution you provide or own their own).
See BitcoinJS's example for creating a 2-of-3 P2SH address, creating a transanction (to fund your multisig address) and spending from a 2-of-2 multisig address (you will need to modify to spend from a 2-of-3 address).
Bitcore by Bitpay (which Copay is built on) can also create multisig addresses and sign/create transanctions: https://github.com/bitpay/bitcore. You can use it in the browser with bower for easy setup, and their API is very well documented. You can see their example for generating a multisig address.
With all of that said, be very careful with your implementation. This is people's money and you don't want to be introducing vulnerabilities that loses people their money (or yours). If you don't feel comfortable with the security implications of your design decisions, create example demonstration versions and have them well vetted by more experienced people before bringing them into production.