Use escrow at all times when transacting with people you do not know and trust. Here is a list of forum escrows:
https://bitcointalk.org/index.php?topic=2439910Never send first, not even incrementally. Do not use reversible payment methods such as PayPal, Skrill or Venmo. They are easily challenged and disputed which often leads to funds being reversed or blocked. Escrow can only protect if you use a permanent or irreversible payment method. Remember this.
A screen shot or picture of the payment receipt is not a final and conclusive form of payment. You must have cleared and spendable funds (cash) or a positive account balance (if using SWIFT wire) before releasing the coins. Do not allow anyone to pressure you to instruct escrow to release the coins based alone a screen shot of wired payment, an MTCN, a reference number or any kind of document purporting to prove a successfully processed payment.