There must be some running gag out there that the fake ones are actually better than the real ones. Ha! But yeah, not recommended to buy from those sites. As much crap as they get around here, Beats are very much a premium headphone and you will pay top dollar for the real ones.
I have a big head :-) I've used iPods, C421, E09k, and home stereo receivers all with great results on the K550. I've never understood the "thin" description. They have been my go to closed HP for a while. Never had an issue with seal. Maybe I just have the right head for them.