As you are the seller, you need his email address, using that, you then send the buyer an invoice - the buyer pays - once you have the funds in your account balance, you can then ship the item - make sure that have the tracking details so you can complete the transaction details on your end.
Craigs List is a wonderful place for things to go wrong -
* If someone wants your email address so they can pay you - that's a red flag.
* lf someone wants offers to pay you more than you're asking - that's a red flag.
* If someone wants you to use another payment service like Western Union - that's a red flag.
* If someone wants to use their own delivery service or wants to pick-up the item - that's a red flag.
If you're selling an item that can't be shipped, tracked and insured (if applicable) - then sell local for "cash".
