Sometimes instead of selling Rope Nets or Perandus Coins the seller will attempt to enter your map and carve your monster or steal that sweet Cadiro deal you have up.
Often they work in pairs — while you're "buying" currency somebody else from the seller's party enters your map and steals the deal.
You have multiple options to combat this behavior:
— Option A) Instead of inviting the seller to the party, invite them to your friend list. Then you can right click them in your friends list and "Visit Hideout". Since they aren't in your party they won't be able to enter any of your portals.
Really, just stop reading here and do that. Don't bother with the other options unless you know what you're doing.
— Option B) You can disallow party members from entering your hideout. When in hideout, press the "Hideout" button located in the bottom. Then uncheck "Allow party members to visit your hideout".
Note that Option B only works if you're mapping from your hideout. The seller will still be able to enter your map if it's open from the laboratory, or enter your other non-map instances. As such, Option A is recommended.
Note that your hideout settings won't apply instantly due to synchronization delay between different servers. As such, Option A is strongly recommended, unless you've had "Allow party members to visit your hideout" unchecked for a while.
— Option C) You can try to block your map portal with hideout decorations. Some of them can be bypassed, so this requires rigorous testing. In addition, it might be possible for people to lag through your decorations and enter your portal anyway. Seriously, just use the Option A it's amazing and works in every case.