I think there a couple of things to think about. If you just want to get rid of as much of it as quickly as possible and are less concerned about maximizing the value, something like ebay is your best option. However, even in that case, I think spending some time researching prices on Bricklink would help you know what prices to aim for. The secondary lego market is very active these days. Even if you took the average current price for each set on Bricklink and added it all together as a single price to sell all of it, I bet you'd find a buyer. If not, knock 5 or 10 percent off that price and for sure someone will snap it up.
If you want to maximize the value and don't mind spending more time managing the inventory, packing, and shipping, you could set up a Bricklink (or BrickOwl) store and put everything you want to sell up there. You'd probably be selling them a few at a time. That involves building an inventory of the sets and keeping that inventory updated as you sell them off.
In all cases, you'll need to distinguish between the different states (sealed box, built and rebagged, etc) in order to figure out the right price.
This article talks about an average increase in the value of lego sets of 12% per year since 2000, so I hope you'll be pleasantly surprised by the value of what you have.
Thank you. I followed the advice in the Editing Part Photos help page, which was quite easy to follow. My photos show the head attached to a body, instead of a free-floating head, and thought about cropping the neck also. But my Gimp skills were lacking - I did not manage to erase the neck in a way that was non-ovbious. So I uploaded them as is. Hope that's OK.
I intend to upload the other heads that I own in the same pose, because I got really confused trying to tell them apart just from the title and Element images. 🙂