I'm a vintage Mac collector. My first computer was a Mac Plus, then we moved on to an LCII. My first laptop was PowerBook 165 (which I got for $25). I mastered System 7 at a young age.
For $15, you did well! Working PowerBooks in good condition tend to sell for much higher.
The number of vintage Mac collectors has increased slightly over the years. People are getting back into vintage hardware. Not for serious work, but for fun in their spare time. So, have some fun with it! System 7 was a unique platform. Learn all you can about it. There's a site called the Macintosh Garden where you can get a whole lot of "abandon-ware". Try out After Dark, Office, Claris applications, games (Crystal Crazy, Spectre Supreme, Hellcats). If you like playing around with old hardware, you'll love it. No OS is easier to network than System 7 - AppleTalk worked great.
The 5300c had a 10.4" 640x480 pixel active matrix color LCD display. From passive matrix to active, the price would have jumped about $1,000! The screen quality on early color (active matrix) PowerBooks is better than on some laptops today, and active matrix is what collectors want, because it lasts a lot longer.
Once again, well done. Unfortunately, my 5300c no longer boots.
I'm no longer a Mac person. As soon as I experienced Windows, I saw it's advantages over System 7/Mac OS (Start menu/taskbar/right-click mouse button menu-scroll wheel/pre-emptive multitasking/memory protection). I dislike Mac OS X, mainly because of it's poor window management, and also the fact that it's a hog on system resources.