One should be very wary indeed when paying considerable amounts of money in order to obtain a qualification.

It is possible to buy qualifications (including PhD's) from 'virtual institutions' off the internet. I think the universities are 'legitimate' in the sense that they are able to do this (there are no laws preventing them). The pieces of paper are, of course, worthless when it comes to obtaining a job that requires a PhD, however.

Firstly: Because you won't have supervisors to write you your reference.

Secondly: Because you won't have writing samples, laboratory results, experience with patients etc.

Thirdly: Because you won't have taken any courses.

One should be wary of funding ones PhD oneself, too. It is hard to gain entry... But if you don't gain funding (for tuition and a living stipend even if it requires TAing or working in the field) then I think it is fair to conclude that your field is telling you something...