As a former seller on MPGH, I mainly used Crypto because people couldn't chargeback. So it meant I could accept orders from noobs (noobs usually tend to scam sellers by charge backing). Not saying being a noob is bad, but noobs aren't trusted. If they get banned on MPGH, they don't care because they haven't built their account. Another big reason was that Sellers PayPal accounts get limited a lot. It becomes annoying to deal with that process. Meaning a lot of money gets stuck in a PayPal account forever.
You can find price conversion of USD to BTC here: https://preev.com/
Cryptocurrency prices are unstable (as we are in a 50%+ drop). However, people believe in the long run 1 BTC will be worth 100k+. Sellers who don't keep their BTC, most likely exchange their BTC for USD right away using a trusted exchange company. Even though it seems stupid that a customer converted their USD to BTC and the seller just ended up converted the BTC back to USD, it still means the seller's PayPal will not get limited and the customer cannot chargeback.
How to buy BTC? Well you can use the MPGH forum to buy BTC using PayPal. There is usually a 15-20% fee. Otherwise, you can use a trusted exchange. You have to tell us what country you are in, before we can tell you an exchange you can use. The most popular real life dollars to BTC exchange, available in the most countries would be Coinbase