Some trade their Items for BTB instead of AC bells.
How you trade like that?
How do I give bells to someone else and have the guarantee I will have the items?
This will explain how to send TBT to another person.
Usually how it works is you visit their town/they visit yours, the person with the items drops them, and before or after the second person picks them up, they send the bells. If they leave without paying, they might pay you after they get the items. I've had that happen many times when payment came awhile after the trade, they might be busy or get sidetracked. This forum is generally 99.99% safe when it comes to trading, so you shouldn't have any worries! If they don't pay you and you're concerned about a scam, you have a few options: flick the switch (if you're paranoid, I tell people pay before they leave so that way it ensures no scam), tell an admin/mod, and send a negative wi-fi rating.