Hi, seashells are given to newer members so they can get a few basic items. You can spend them in the Kapp'n's Seaside Shack tab of the shop. They cannot be traded, however.
if you joined tbtf after a certain time frame (october 2015), then you automatically get 10 seashells. new members used to get welcome bells (iirc 800 tbt) but it was discontinued and then the mods introduced seashells.
you can spend your seashells in shop > kapp'n's seaside shack for certain add-ons (i.e. username change, mailbox expansions) but you can't spend them in the bell tree shop for collecticles, groups, bigger mailboxes, and so forth
here's the direct that introduced seashells! scroll down to new member currency if you want more info
