I am glad to hear you have decided to keep an eye open for a used bike shop quality bike rather than buy a big box store model. I have made it a personal mission of mine to help my friends, family and even strangers ride quality bikes instead of buying the junk they sell at places like Target, Walmart, etc.

I guess I am what you call a "flipper" - finding bikes at thrift stores, garage sales, craigslist, etc. and fixing them to turn around and sell for a profit to pass on to a friend, family member, etc. I have pulled bikes out of the garbage, from city clean up days, etc. that have gone on to a whole new life with a new rider. As you said, many of these bikes were barely even ridden and spent a majority of their lives hanging from the ceiling or in the corner of the shed. If you have a little patience and start looking around you will be very suprised at the quality of bike you can pick up for little - or even no money! If you are looking for a used rig here are some tips:

1. The bike should have alloy rims - no steel!
2. The bike should have downtube shifters, brifters or barcons (barend shifters) and not the stem mounted ones.
3. The bike frame should be labeled with some sort of indicator of the tubeset - ie Columbus, Reynolds, Ishiwata, etc. and should be butted, double butted, triple butted, etc. These are generally indicators of a quality bike frame. Often the fork is also labeled with this info and that is even better.
4. The brake levers should not have "suicide levers" - those extra levers that allow you to brake from the horizontal portion of the bars. These are generally only used on lower end models.
5. Components including crankset, brakes, bars, stem, seatpost, etc should be aluminum and not steel.
6. The crankset should not be cottered and should have aluminum rings.

I have flipped dozens of bikes over the past few years and I try to stick to these general rules when picking up bikes to pass on. Obviously some will disagree and point out that old Schwinn's were built like tanks and last forever - but these same bikes are also extremely heavy and much more work to ride around in my opinion. Anyway, I hope this helps. Good luck.