I had sold 2 PSA Aarons on ebay same day; mixed up the order, both communicated to me I got it wrong, ask each other to send to each other or return which ever easier; they send send to each other; I paid for labels and emailed them to each one, The one guy sent to other right away, the other person waited like 3 weeks to do so.
Not a pleasant experience; Hence I double check before I seal envelope and usually twice now when hungover.