I think that craiglist is the best way to go for larger things, although the flakiness is no fun. You just have to persevere. I find it easier to put my phone number and have people call me to sell something-- it is easy to send a quick email and then flake out and not really want to buy the item, but to pick up the phone and call is a bigger commitment in my very humble opinion. Plus then I can screen then based on their voice and see if I want them coming to my house, or if I want to pick another place to sell the item.
Diaperswappers.com is a great place for selling cloth diapers and clothing.
I didn't care for threadup. I don't like not being able to pick each item, and it is kind of expensive for second hand stuff. I like being able to go in to a consignment store. Many of them here in our city I have a lot of credit... so I can a child and shop, and if we find something, great, if not, well, no biggie.
I guess Amazon might be an option now as well. But I've never sold stuff on Amazon. Ebay is great--I find that offering free shipping is helpful to attract attention since so many people pad the shipping like crazy. Although it is good to put the shipping cost into the price (start a bit higher). Unless it is a really heavy thing, then I will have exact shipping.
Offer to ship to Canada is good, too.
I would never, personally, but a used car seat, but know people who do.