I've sold to KEH a few times. You know you're going to only get wholesale prices, but you don't have the risk and hassle of selling yourself. That's sort of the tradeoff. As for how long it takes, in my experience their inspection takes a few days after receipt, depending on how busy they are. If you are trading it in on something else, you get an extra 10 percent, as I recall, and it's all offset against your purchase price. Otherwise, they mail you a check, which also might take a few days.
I called in and spoke to a buyer to get my estimate, then I could call her directly to keep track of the transaction. That helped when one camera was delayed in inspection because they (wrongly) thought there was a problem.