use an escrow service to avoid scammers.
what's escrow?
Escrow meaning you find a very trusted member to be the mediator between you and the person you want to exchange.
You send BTC to escrow, other person sends LTC to escrow, escrow sends you LTC and him BTC.
The person might charge a fee.
It's not necessary for the escrow provider to touch both sides of the trade if delivery of one or the other side of the trade can be established beyond doubt.
So for example:
Buyer sends payment to escrow
Escrow tells seller payment is received
Seller sends LTC to buyer
Buyer tells escrow that goods are received
Escrow releases funds to seller
Given that the block chain can track all steps here, the chances of someone scamming and convincing the escrow to release funds to the wrong party are small to nonexistent.