There isn't a built-in feature to make a shop, but you could try a few different methods.
1. Simply keep the stuff you want to sell with you, and occasionally put into the chat that you have stuff to sell. If someone wants to buy it, send a tp request.
2. Use a redstone system (the one iri_pone uses is very popular) to sell items to players when you're offline.
3. Create a shop building (like Tinker's Trinkets, for example) and keep item frames/signs with items and prices, keep the actual items somewhere else, and give the items to players when they give you payment.
Hope this helps!
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