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I agree that it would be fairly simply to replace the xoops_banner smarty variable with output from another script, and just provide a link in a menu to the 'backend' of the banner ad software where people can purchase ads etc.
However, I can also understand that some people like features fully integrated in Xoops. Often it is just to prevent the need for logging in twice, but also it is to give the whole site a more consistent professional feel, rather than something that is hacked together. This may seem like a relatively small job, but from personal experience it takes a long time to do a good job as you need to have thorough knowledge of the code you are integrating (typically this is the hard part because much PHP software is badly written/organized).
Perhaps an 'advanced' banner management module exists for Xoops... I don't know. I believe the module development team is taking over the banners functionality, so maybe you could ask at the module development site (there is a link icon in right column of this site).
Most modules arise because of a personal need for a particular function, and usually that is incentive enough for someone to write a module. vB3Dev, if you are a coder I would suggest starting this project yourself as it would be a good way to start becoming familiar with the XOOPS core and if you wrote the banner management software you are way ahead of everyone else in understanding how that works. If you are not, and if you need this quickly, I would offer some kind of incentive for someone to do it for you.