Things people collect, things you suspect people might Want to collect one day, things that are in general demand, and things you can find a good deal on.
I've bought from CL (with the intention of selling on eBay/Amazon) so much, I rent a desk at a local pawnshop and just schedule times for people to bring me their stuff.
I usually make a good profit from older video games, action figures, and other little knick-knacks. I suspected a couple years ago that the "Turbo Man" Action figure would be in demand, because I think that in time, people will consider the movie "Jingle All the Way" a classic. I target searched for them through CL, paid for shipping blindly, and I've bought those things for $30 and sold them for $200 before. I sell video games because it's easy volume on eBay, and volume and traffic is essential to eBay. Another great thing is to offer people money for name brand clothing. I cant tell you how many pairs of lightly worn Ugg Boots Ive bought for $15 and sold for $50 or more. I'm still reselling them during the summer. Just put a CL ad up saying "Buying Name Brand Clothing, Shoes, Purses, and more" - communicate through e-mail and try to get as much information as possible. You'll usually get a list of clothes people want to sell off to you. Do a quick "Advanced Search" on eBay to figure out what they sell for, and offer the customer 20-25% of that price. I can't tell you how many Abercrombie shirts I've bought for a couple bucks and sold for near retail price. Plus, name brand stuff is an AWESOME way to bring traffic to your existing eBay inventory.