Why might MultiBit not show a transaction that was sent to me?
You can ask the vendor for the transaction id of the transaction where they sent the bitcoin to you. Look it up on https://blockchain.info to check it exists and is sending bitcoin to your receiving address correctly. Look at the date of the transaction. Go into MultiBit, select your wallet and choose the menu option 'Tools | Reset blockchain and transactions'. Reset from the day before the transaction. Once it has resynced to the network you should see it in MultiBit.