Will1335 :
I am looking for a cheap beginner camera preferriy under/around the $400 mark. I am mainly looking at used gear off of eBay. I've mainly been looking at the Nikon D3300, D3400, Canon SL1, and the Sony a6000, or a5100. I'm not really sure if a Mirrorless would be better for a first camera, or should I just stick with the cheaper DSLR's? Also what if a camera comes with 2 lenses for a similar price to a (better) different camera with only 1 lense?
Most extra lenses on ebay are junk. 1 good lens is better than 2 bad ones.
Between the types of cameras... DSLRs are generally easier to learn to take good photos on. Nikon and Canon have the best lens selections if you decide to add more lenses. The Sony's focus faster in video. The ergonomics of the DSLRs is generally better. The physical dimensions of the mirrorless tend to be smaller.
Among those DSLRs...
The Nikon's tend to get better reviews and with the same quality lens, the Nikon will produce an image which is less noisy and has more dynamic range. The Sony splits the difference between the Nikon and the Canon, being closer to the Nikon.
With the Canon lenses, to start with make sure the lenses have IS (canon for stabilization)
With the Nikon lenses, to start with make sure the lenses have VR (Nikon for stabilization)
I've owned all three brands. I still own a Sony and a Nikon. I prefer Nikon. I use Panasonic and Sony for most video, Nikon for everything else.
The Canon you're looking at does not have dual-phase detection and has the slowest AF of the bodies you are talking about.
The comment about limited DX lenses for the Nikon is poppycock. Nikon has the largest lens collection (followed closely by Canon). The DX bodies can shoot FX lenses just fine.
The worst UI is in the Olympus cameras, you're not considering one but Schwebbz mentioned them.