Some sample pseudocode using the new Accounts based commands in 0.3.18.

print "send to " + getaccountaddress(username) + " to fund your account"
print "balance: " + getbalance(username, 0)
print "available balance: " + getbalance(username, 6)

// if you make a sale, move the money from their account to your "" account
if (move(username, "", amount, 6, "purchased item"))
    SendTheGoods()

// withdrawal
sendfrom(username, bitcoinaddress, amount, 6, "withdrawal by user")

You can use listtransactions(username) to show them a list of their recent transactions.