Shopstr.store
#Amethyst has it built in, as well.
Need more folks to post stuff to it.
Discussion
Shopstr looks very rudimentary. I had to close it when it caused my extension to spam me with requests to decrypt messages.
Interesting. I don't have that issue on shopstr, but I do on the plebian marketplace. It's trying to decrypt your DMs for you. If you have quite a few, it will spam the requests for each one you have ever received.