I own a website that does e-commerce. On my site, I allow other websites to place banner adds. If someone leaves my site and hops onto another site and then purchases merchandise (as the result of first visiting my site), how can I track the sales for my commission portion? I thought there was a “follow me anywhere” ability so I could audit the trail leading to sales? Please help!

If I were you, I would require that advertisers allow you access to their own tracking system so that you can verify sales that came from your site.

... just my 1.5 cents worth...