Cheng: You might want to keep your spare grandma in case you decide to change outfits for your avatar. I have three already because eh grandparents are my favorite 123 figures. I love their gray hair and have several regular Klicky grandparents as well. Wish that PM sold the hair pieces through DS because I would do a lot of new figures with extra hair.
We do find some used 123, but more often regular PM, in garage sales (held at homes usually on the driveways), on craigslist (online free listings in a particular area and quiet effective for selling items you don't need), and of course on eBay. Sometimes there are sets in the second hand stores run by Goodwill, the Salvation Army, churches, and other charities.
The problem with used 123 is that the pieces usually are chewed up by teething kids. And the rough treatment is hard on all small toys. So I tend to pass on used 123.
I wish that PM would make the DS part numbers more available and would offer more 123. It is getting hard to find in stores here; our local toy store is the only one that carries much of the 123 line in our area with a population of about half a million. It is nice seeing what you are buying before you get it. So I tend to buy it from the local guys.
Glad that you appreciate 123, Pgal