Blueprint can be thought of as a scripting language (like JavaScript or LUA or whatever.) Thus, you can build a trading system with Blueprint for the front end.
Building the entire trading system in Blueprint isn’t really possible, even if you can get everyone to connect to the same server, because things like “atomic commit to file” which are important to avoid item duplication or other trade corruption are not part of the Unreal Engine.
Thus, you probably want to build your trading system as a web service, similar to any other web site (but as an API, without fancy web graphics) on top of an application server and a database.
However, there are no web services nodes available for Blueprint. Thus, you’d probably need a programmer to implement Blueprint nodes that expose your trading system. Then you can call those from Blueprint.