^ What he said.
Unless you have the tools, and are an experienced carpenter or DIYer, don't try to install slab doors.
Regarding the "real lumberyard" versus "apparently fake lumberyard" debate... You will get what you pay for. If you are framing in the wall, and do it correctly, you can get standard sized pre-hung doors from any of the box stores - doors that will be perfectly fine, and very easy to install. You'll be able to find doors in all styles and price ranges. If you want cheap, you can get them. If you want expensive solid-core doors, you can get them too.
If you need to retrofit an off-standard door, you won't find them at the box stores.
The doors being referred to are probably the stain grade solid pine doors-at HD.They only come in slabs, not pre-hung; guess they are meant more for retrofits.