I mostly sell on Amazon..although it's mystifying to me why they only allow new items in several areas. I'd LOVE to buy used toys or baby products! I keep meaning to sell on ebay, but from what you say, it'd be annoying.. I tried it once and set things up, but I never seem to get around to adding that to my activities.
I do use freecycle all the time though, and have had overall *excellent* experiences. And I get great stuff there as well because I've built a good reputation. But that's the only "compensation" of course. I do sell successfully on craigslist.
The other place we do well frankly is the citywide yard sale in our town. But of course you have to store up your stuff for that. It paid last year for our swingset and this year for our pool cover.
Amazon is great for books, CDs etc, but only if you can sell for $5 or more (slightly less for light items like CDs).. otherwise it just isn't worth it.
I just did an article on Suite101 on this if you are interested, Lisa. Also, swapping online is great if you are interested in getting other items in exchange. I did articles on Suite on the top 3 sites I use. This is how I get much of what I review or refer to for breastfeeding and early childhood.
Here's my Suite Profile which has all those articles there...
Nicki's Suite101 profile and articles Hope that helps!