Find out what it costs to overhaul (inc repack hubs/bb) a bike at your LBS (probably $150 $200). Add that to the price of the used bike when comparing it to a new bike. New bikes also have guarantees and usually a free check up.
Used bikes are only a 'bargain' for people who do their own work and do not include the value of their time and tools in the price.
Turn the bike over put your thumb on the fork a mm or 2 away from the rim, spin the rim. If the rim is not true, your LBS will charge about $20, assuming the rim is not shot. spokes should be tight. Was the presta valve hole 'expanded to fit schreader valves? I would not be surprised if the cones were shot ($10 per axel assuming you get the cheapest). Spoke nipples stripped by spoke wrench.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.