Hi Trace,
That 550 Maxim is a cute little bike. Was tempted to pick one up for the wife a coupe of years ago. Looks like lots of fun without sinking a lot of cash into a bike. $500 sounds like a good deal for a bike in apparently good shape. Around Calgary, these go for $1000-2000 CAN in decent shape.
1. The only real trouble with carbs is on the bikes that have been sitting for years without proper storage steps taken. You know how that varnish loves to build up and plug all the little jets and passages in a carb. But if you're even a little mechanically inclined, rebuilding carbs is a snap. I bought a rebuild kit from oldbikebarn.com . I've been avoiding Ebay having been burnt on my last few purchases, and found that you're not really saving much anyways. Plenty of places online that sell parts for these old bikes. Just about anything can be had now! Check out Mike's homemade YICS tool. The guy's amazing, and it's worth having. Getting one from the Yamarobber (if they still have any) costs many times more and isn't any better than Mike's. Go see
http://www.xjowners.com/ if you haven't already. Advise synching the carbs after cleaning them as you're defeating the purpose of the maintenance without balancing them afterwards. A $60 set of mercury gauges and 30 mins of your time is just the ticket.
2. See above
3. Values depend on where you are. Late '70's and early '80's bikes haven't really hit the collector market yet, so we're all just riding "old bikes" right now. I bought my 750 Seca during a transit strike, and paid 25-50% more because of it.
Hope that helps!
GG