A puzzle problem involving coinage

A shopkeeper and her customer each have an unlimited number of coins.
However, they are of only two denominations 3 and 5.

1. What amount purchases are not possible using only these two denominations of
coinage, if the shopkeeper is allowed to give change back to the customer?

2. If the shopkeeper has no coins at all to give as change, but the customer has an
unlimited number of these two coins, what purchase amounts are possible?