Let me be clear and say that my intention is not to screw over anyone or lie about how my site makes money. It's a typical 'made for adsense' site, but I've found a source of paid traffic that has yielded me some nice revenue. Is this something I can sell? I wouldn't mind selling my source along...
1) This is usually mentioned as part of the outreach process.
2) 99% of the time you will just send them your content.
3) Paypal is standard, as is payment up front. Repeat customers might get some leeway on this.
4) Ideally a payment method that you can flag/charge back in case of content removal.