Yeah, the double trade glitch happens when you accept trades really fast. I've had it happen and I just always send the item to the person who bought it.

Yeah, that's just uncool to purposely use the glitch in the trades to your advantage.

I sometimes disappear from my board, but I usually always come back and check it and when I forget, I comment the user back.

TBH, the best solution is to update your FIRST POST as a seller stating.

Oh hey going afk for a bit BBL to finalize deals!

And then go off and do whatever, same can be done with buyers...leave a comment saying you're going out somewhere and you made a trade with your offer or something (if its not sp) and let the seller offer their shit on it.