The solution is for SE to include a lot of customized /tell filters and filters on friend requests like not allowing someone below certain levels to friend / tell you and not allowing people who aren't on your friends list to /tell you or in your fc to /tell you or /friend you.

The list goes on really. There's so many ideas for customization that can make it so we only get the /tells we actually give a damn about then these gil sellers can simply bugger off.