I'm considering buying an older Advantage 20.5 Bowrider. Are there any common issues with stringer/transom rot? Everything I can see looks good. Floor is solid. Boat has been stored outside with cover. I am not familiar with this brand's structural history.

It's a west coast boat. Go to riverdavesplace.com lots of info there.

I haven't heard of any transom rot with Advantages, but that will 100% depend on how the boat was kept. Any boat is susceptible to transom or stringer rot. Get a survey done on it, they use a moisture meter and can tell you.