If you have a popular gun (Glock, Sig, XD, HK, beretta, S&W, etc) and price it right it will go Fast (under 6 hours)on KSL. If you overprice it then it will stick around for 1-3 weeks before sale. If you WAY over price it then it will be there until you pull the ad.
If you have a no-name (or just uncommon) gun then you might want to trade it in unless it is less than $200. I would still post it on KSL and see... but after a few days the trade in starts looking good.
I had a buddy with a glock 19, trade in value: $285, KSL (sold in under 15 minutes) $395
Same guy had a Starfire (forget the model), Trade in value: $195, KSL (almost 90 days later, listed fror $250) $225