I am also a certified residential real estate appraiser. What I would do as an appraiser is to find the three most comparable (as far as square footage, # of bedrooms, # of baths, condition, etc...) condo sales in as close proximity as possible to yours. Ideally, lenders are looking for comps within a 3-month time frame and 1-mile radius. That's not always possible. Especially these days! For condos, we always want to find a comp within the complex. Yours only has 3 units, so I would be inclined to use that sale from back in September as well. I believe Trulia offers fairly reliable recent sales information you can comb through or head down to the Lowell Assessor's office to research all recent sales.
Your best bet is to find three recent sales of "similar" units in your neighborhood (either through Trulia, Zillow or the Lowell Assessor) and then factor in the recent sale in your building. Was his sale a true, arm's length market sale or was it influenced by other forces (foreclosure, pre-foreclosure, sold to a family member, etc....)?
A little research on MLS shows just one condo in Lowell selling for $239,500 in 2006. I think this might be yours (?) and I'm happy to discuss it further with you if you'd like (617-828-4127).
Good luck with everything and I hope this helps!