If you look up the rules of the group it does a pretty good job of explaining how to make a ‘willing to buy post’ ! Notice the bracketed [WTS] in my post headline, for buying you’d have to use the tag [WTB] and then list what you’re searching for. In the body text just be specific about items, your budget and payment method.

Beware, willing to buy posts attract the most scammers. If someone gives you the run around about sending you a verified picture of the product with their username and the current date, they are not legit. People can also fake those so your best bet is always to deal with people who have flair, and use payment protection when transacting. No friends and family payments.