Location, location, location...
If I could rent in Waltham in the $900's with heat and hot water included, why would I drive farther out for the privilege of owning a condo in a city with a long commute and a poor quality school district? It may be cheaper to buy there in a hot market, but it will be a lot harder to sell in a balanced market or worse, a buyer's market. If it's not moving in a hot market, it's likely overpriced and in a location that's not highly valued by the market. In your shoes, I would listen to your agent and pass on this unit.
The assessed tax value is meaningless. Look at recent comparable sales to estimate value. If it could be had for a "bargain" price, it would have sold. The sellers are probably unrealistic, especially since this is their second try.