Best option for selling banner space

Within a free B2B web application, what are the best options to sell banner space?

Can’t use Adsense as the app is password protected and they don’t allow this plus I do not want to allow competitors to target my users so what are the other options? Have looked at buysellads as one option but are there others worth considering?

I know I could do this manually but where do I find buyers if this is the case? Also, from experience, is it worth the grief or is it better to pay someone a % to do all the legwork for you?