as far as merchandise, it's usually old style tee shirts,pins, jewelery, toys and outdated souveniers.
The stuff is really a great deal! I like those die cast attraction toys, they usually sell for $2.99 at the CP and $5.99 at WDW.
After Christmas they usually have a lot of the Christmas stuff that didn't sell at the parks. This time of year you'll probably find Thanksgiving stuff.