The safest & fastest way would be to look at the "Bells" section under that user's avatar on the sidebar.
>Click on those numbers (for example: you have 410 as of now)
>A pop up menu will come up, and you can enter the amount of bells, along with notes or comments as a record, if you like.
Both of you will receive a transaction receipt instantly,
so it's safe, fast & handy.
Another way would be click on that "Currency" tab > 'Bells"> from that pull down menu.
BUT the catch is you have to enter the correct user name yourself, and sometimes that's where an error would occur,
as some users name might be v similar to each other.
For converting / selling your forum currency to in game bells,
you can go to The TBT Marketplace:
http://www.belltreeforums.com/forumdisplay.php?69-TBT-Marketplace
You can start a new thread there, or try one of the several shops/sellers that would buy them from you.
The current conversion rate is *about* 100 BTB to 5-6 mil ingame bells.
http://www.belltreeforums.com/showt...ate-from-TBT-Bells-to-AC-Bells-and-Vice-Versa
Have fun! C: