I just looked at craigslist for Vermont, and apparently there are a few great looking honda cbs of various sizes for what I would consider extremely good prices IE under $1500. I live in Texas now (and CA until recently) and it's virtually impossible to find a 550 in decent shape for anywhere near $1500! Guess I'll have to move to Vermont!
Regarding your question, judging by my non-scientific research, it's a regional thing... as of RIGHT NOW most of the 70s hondas on craigslist in Vermont are sub-$1200, though they probably aren't as nice and original as yours are.
Also, in terms of making your ad more appealing, you absolutely *MUST* put good, current photos up! Pictures are worth a thousand words, especially when it comes to getting someone excited about making a trip out to check out a sweet, sweet vintage Honda!
Just my two cents! Good luck!