I did a search on eBay. Apparently you have to use the word "overlock" in the search, as "serger" doesn't come up with much. Couldn't find your particular model, but did find the following Pfaff's in the completed section:
http://www.ebay.com/sch/i.html?_nkw=...&LH_Complete=1
Only those with a price in green actually sold. Might at least give you a ballpark, depending on the features your serger has. Prices at garage sales tend to be much less than on eBay. Can you demonstrate at the garage sale that the serger actually works?