I used buyselladds some time ago for a couple of my old AD files.
If you want to take the most of the ads, you have to carefully choose the media, not only because the different banner prices (some huge), but also for targeting. I mean, if your intention is to promote the files, think of readers, etc. But if your objetive are referals, then choose sites not related with design/dev, for example.
I can't tell you exactly which sales came from the banners (Envato analytics tool is just annoying imho) but anyway the expense was not high at all, and i guess just with referals i got was worth it.
Apart from this, Buysellads site and authoring system is pretty well built.