(1) use money to buy something (Money leaves wallet)
(2) sell something to earn money (Money enters wallet)

buys <userid, amount>
sells <userid, amount>
transaction <transactionid, userid, amount, type>

i.e.
insert <u1, 90> into buys trigger insert <t1, u1, 90, 'buy'>
for example

buys <userid, transactionid>
sells <userid, transactionid>
transaction <transactionid, amount>

i.e.
insert <t1, 90> into transaction and insert <u1, t1> into buys