Yes, legit charities are pretty up front about what percentage of your donation goes to the cause and what percentage they take for operating costs (which includes salaries). If this info isn't public, I'd be super suspicious of their real motives.

If you suck, you can make personal purchases and pretend they are part of what the nonprofit needs to operate, but that is illegal and if you get audited there will be legal consequences. If you're buying a pack of pens or printer paper or something, you're going to get away with it, but if you use nonprofit funds to buy yourself a car or pay your electric bill at your house, neither of which is necessary for the charity to operate, you risk some big trouble with the IRS if they decide to audit you.