As soon as your bitcoin node receives the signed transaction (should be seconds), approve the transaction, but listen for other spends of those same coins. After a few seconds, the cost of double spending those coins would be basically the same as the reward for mining a block, so it's unlikely that someone would defraud you for something worth less than ~$2,000.

There's almost nothing you could be selling online that couldn't be reversed within an hour or so of the sale in the relatively rare cases of fraud, with the possibly exception of wire transfers of cash.