Best method of purchasing

[Idstumped]

I am pretty new to the site and saw a few items that where interesting to buy but was wondering what the best method to purchase items that are listed on the classifieds buy/sell pages?
How can I be reasonably sure that I will receive an item after paying for it?

How do you pay for an item?

What should I look for to tell if the seller is trustworthy or not?

thanks