You don't have to give everyone the permission to set up signshops. I'm just asking for a permission for me to set some signshops up. The plugin is already there as the spawn shop is using signshop, you'd just need to add me to the permissions for creating one.
I'm not trying to get rich with my shop, I'm not even online that often or know what to do with all the money, I just want to help people. For example there are always people that just enjoy building stuff and not setting up void miners and ore processing machines etc.. They just come to my shop, buy 4000 wood and start building. You can't provide such a service by signshops at spawn or through the player market, not even through player signshop markets (at least not that easily).
Or do it like other servers and create a separate, small world where people can create shops and only allow signshops in there. How can you abused a sign shop though? Selling different items than written on the sign? yeah that could be a problem. But that's why I only ask for myself, not for the whole server.