If you're primarily selling online, I can help you some. I'm going through the process now of getting my business sold.
I spoke with 3 different brokers, my favorite was Quietlight. They were professional, upfront, and had the lowest commission quoted to me at 10%.
A broker functions a lot like a real estate agent. They'll vet the buyers, set up a 3-way call with the serious ones, help go through the process, help draw up agreements, and of course expose your business/site to their network of buyers.
Beware of the broker that will try and hook you into an exclusive arrangement by giving you unrealistic multiples that he could sell your business for. They get you in the exclusive, and then after a month of no one interested, they drop the price back down, but now you're stuck with them for the next 6 months.
Reasons for selling: I have another business I want to get into, no longer interested in the products I sell, and some gut feelings about the future of the market I'm in. I want to get out while things are going well, like you would when selling a house.