Sedo, Afternic, etc. on average will sell, in my best estimate, about 1% or less of the domains they list in a given year, which means about 1 in 100 speculative registrations are resold. I find their service to be useless. You would do just as well to create a landing page indicating that the domain is for sale. As long as you have a clue about value and negotiation you have an equal if not better chance of making a sale.
Choose your domains wisely. For each 1 you sell you have to make up for the 99 you never will sell. With Joe Ordinary junior/newbie domain grabber the stats are more like 1 in 200 and that 1 won't sell for much.
If you are going to play in this game then chances are you will do better by buying an underpriced domain in the aftermarket than by registering the next 100 nifty, catchy, brandable domain names that you can think of.
The problem here is the 'lottery mentality', but it's a game without a winner/loser declaration date. People register domains and hold onto them believing they have a winner. Until they surrender their ticket they believe there's a chance they might win. God forbid they ever sell one: They can become hooked.
Sad observation: I've registered a number of decent domains over the years. I selectively edit my list, dumping ones that I believe that there's only a most marginal chance of a sale. I check on them, post delete, from time to time only to find that another true believer has registered them for their resale potential. It's too bad that when you cut them loose you can't enter a permanent line in their WhoIs record: Abandon hope all ye who enter here.
These are the confessions of a reformed domain accumulator.