IYO: What older frames are generally pretty good that one might find at a yard sale? I am thinking along the lines of Panasonic. ( I think they mostly made decent frames.)

You can get good info about that at the Classic & Vintage forum.

I do a lot of yard sale shopping for bikes and bike stuff. It's hard to find good quality bikes at yard sales. If you know how to build up a bike, you might want to buy any good vintage bikes from yard sales, lube, clean and repair them then flip them on CL. Forward that money to buy a good vintage bike. As for what to buy, if you've got bucks, high end Columbus steel frames are great, Reynolds 531 and Tange Prestige are all top line tubing used in frames. Usually, those are very pricey. Bikes that can be had for under $200 that can be built up into great rides would include frames with Reynolds 501, like the 84 Trek 560 or a Peugeot Ventoux. More common and less in demand are the Japanese frames from the 80s made with Tange 1 or Tange 2 tubing. These are great frames that are relatively plentiful. Better models of Nishiki used Tange 1 and the Prestige model from the mid 80s used Tange 2. Centurion used Tange 1 in their Ironman model bikes that are in demand today. In much less demand is the Tange 2 framed Centurion LeMans typically selling for under $200 in ready to ride condition. If you post over on the C&V, I'm sure others will provide lists of their favorites that are affordable giving you lots of options. Another brand that seems to be popular right now is the oversized tubing Schwinn Paramounts of the early 80s. The series 1, 2 and 3 bikes are selling for $200 or less and make a great platform. I'm in the process of converting mine now.
BTW, there's a lot to be said for building up one of these old frames if you do the work yourself. You can build up a great ride with some newer components that will be a very noticable bike. My 86 Nishiki Prestige, built up with 8 speed Ultegra and Alex deep V wheelset always gets as much attention as the dude who shows up with some $5000+ CF bike, maybe more.