I'm not quite a real authority on this stuff, but I've been around the block a few times.
Far as I can tell, every possibility is awful. I believe Mile High Comics had an invoice system where you basically sent your stuff and they gave you money. That's vastly oversimplified, but they at least have a process. They're Mile High Comics and they can pull off that sort of thing.
Comic shops that aren't Mile High are vast wastelands--if they even buy your stuff they'll go a certain percentage under Overstreet, and most stuff they've already got piles of. They're not doing a bad thing--they're taking on risk and inventory, etc. and you ever noticed how full those back-issue boxes are at the shop? Not much movement.
eBay is okay, but you're looking at lots of time and effort (photos, packaging, etc.) and you still probably won't be too happy about the prices you get. There are some issues that will go for a decent fraction of the Overstreet price, but the buyer is looking at unknowns--is it really the grade you say it is, for example? Selling single issues is usually not worth the effort, except for those very few that you are already aware of. Selling them in lots will probably give you a (comparative) net loss because all of that uncertainty is spread out across 10 or 50 or a full run. It's a fire sale and people spend accordingly, even with some Nice stuff in there.
I understand people sell them at cons. That sounds tedious and risky and I've got 20-something boxes. Though I did see a guy that just set up a booth to (supposedly) sell off his collection. It was a really good collection, but most of the people with a booth brought "stars" from their inventory--big first issues, etc.
Nostalgia helps some, too--hard to get out in front of it properly, but new attention on some properties bumps the price some--expands the market some.
I've been tempted to sell entire runs I have on eBay. After 20-something years in boxes some of my stuff is almost worth trying to sell, but oddly not the stuff I *expected* to be able to sell 20 years ago. For example, my copy of New Mutants #98 is hhhuge but my Jim Lee X-Men are pretty shrugworthy. Never expected that. The 90's were a terrible time to try and be a collector, and it's not getting better, except maybe at the very high end, where some of this stuff is now legit pop history on its own.
Or maybe I'm full of carp. Meanwhile... what you got? I got gaps in my Tomb of Dracula run.
Rupe