When I'm selling these days, it generally boils down to craigslist or Ebay. I'll go ebay first typically for things that either need a wider market than my local craigslist (Wash. DC, which is huge) provides and/or the shipping isn't a hassle or so expensive that it eats into margin. Generally speaking, Ebay will usually get your highest price since it potentially easily reaches a worldwide audience.

Here are how recent sales of some of my stuff broke down among the two venues:

Craigslist: Bulky, low buckage kids stuff (stroller, car seat, crib, etc), golf clubs, bikes (whole/frames).
Ebay: Bike parts, guitars, guitar parts, clocks, woodworking tools.

I actually rather prefer ebay, since you spend less time on phone calls, no-shows, face to face salesmanship, and default to that wherever possible. I think a lot of other people make similar decisions, which skews craigslist towards items where shipping is a hassle, generally objects that are fairly large in relation to their dollar value. My one attempt at selling a high dollar/size ratio item (a very nice guitar) on craigslist was pretty unsatisfactory, got virtually no responses, then sold it on ebay in no time for more than I listed it on craigslist.

I think that craigslist gets similarly used by buyers in a resulting feedback loop - people look there for lawnmowers, old vacuum cleaners, etc, and go off to ebay for items that have a higher value density, or are rarer, or for a wider selection. However, because of this dynamic, you actually stand a better chance of obtaining an item at an undermarket value than on ebay, where pretty much everything usually closes at or near market value. While I've been satisfied with the bikes I've purchased on ebay, I've gotten much better deals on craigslist, but it also has been more time trying to catch the deal falling through the cracks there.

Obviously, there are plenty of examples to the contrary, this is all just some generalities I've observed/experienced.