Has anyone purchased points form themileageclub.com website? Is this a safe thing to do? Do the airlines accept these points?
No experience, but a few different businesses like this exist. Normally, if someone is selling points outside official channels it is not sanctioned by the airline and you are breaking their terms and conditions by buying them and using them. That can lead to closure of your account and cancellation of any tickets you have bought with them - if they find out, of course.
It’s definitely not risk free, and though some of the businesses I have heard will try to make good on any cancelled tickets with refunds/other tickets, I don’t think there’s any guarantees you can expect this to go smoothly.
I have removed your other question about other program miles purchases to keep this question on topic. In short, there are few ways to buy miles with those programs, Starwood purchases and transfers are likely your best option.