Sales in my experience are mixed. You can do very well on eBay, much better than trade-ins, but other times not so much. eBay takes 10% plus $0.30c from the total price, and most of the seller payment methods take another 2-4% cut. In the end you typically get about what most companies will take as a trade, unless you have a mint copy and are lucky enough to get a bidding war going. They can also hold the money for up to 30 days for new sellers, if you haven't been on there and sold a good volume of items. A new seller also cannot choose who can buy from them, so it's hard to block scammer accounts. I for one would not want to be selling high value items on a new account on eBay these days.
Trade-ins are easier, because you can get it done and take your new gear home the same day, but you can get undercut, so if there is more than one camera dealer in town try to get more than one offer. The worst undercuts I've seen is on used accessories, like battery grips and the like. Lenses don't tend to be that bad.