QTH.COM Swap Chat

I know this has probably been said here a few times but I see it more and more on here (QTH) Why would a person list something on here and also list it on ebay and say "sale also pending on ebay" And I say BULL S*** if your going to sell it here then sell it here not on ebay also! I have had good luck here buying and selling but I just cant wrap my head around why someone would do this???
Just my input