Any good tips for buying stuffs on social media safely?

Hi. I'm thinking of adding rife machine on my list for treating parasites/candida. I already bought zapper and it's coming next week. But since rife is too pricy for me, i've been looking for the used ones for sale.

I'm in the middle of buying one on facebook and im very peronoid about buying stuff on the net, especially on SNS. I worry about getting scammed. Any other way i can get the product? How do people sell/buy stuffs safely on facebook? Tweet