The difference between DSLR and point and shoot such as yours is quite a bit more than 'megapixels'. It will also be demanding more precise control, because of the way that dof and f-stops behave on an SLR lens - you will need to be more mindful of your photography.

KEH.com and adorama are the best two places I know of to get used photo gear, I wouldn't trust tristate either.

'ease of use' is a bit of a misnomer, every DSLR under $3k has the same auto modes and the same automations. But the whole point of DSLR is to avoid using those crutches. If you want the actual benefits of an SLR, you need to understand the technology and apply the proper techniques to your photography. Dont get one to just use it like you used your Samsung, that would be like getting a 4 cylinder 4-speed auto transmission ferrari. It just doesn't cut it! For me, 'ease of use' is a priority mode combined with AEL and/or exposure compensation, not throwing on an automatic mode. What it boils down to, learn enough to control a fully manual mode, then you will start getting the pictures you want. I haven't used an auto mode for several years, and quite frankly I'd just prefer they were left off of my cameras.

A great starter would be something with fewer megapixels, they offer much easier workflow and overall experience (you'll find that 6-8 megapixels is a lot less picky than 12-14). Perhaps a Konica Minolta Maxxum 7D, or a Canon Rebel XT, or a Nikon D70... I'd trust a 4 megapixel DSLR before a 16 megapixel point and shoot, don't worry about the number of megapixels so much.