Anyone here know about vintage steel bikes?

[Popoki_Nui]

OK someone near me has a Cierra 12-speed tourer for sale for $40 and a vintage Schwinn Super Sport for $60. I'm asking about the sizes, but ballpark (as I've never *been* on a road bike) if I'm 5'4" and have a 31" inseam, what bike size should I look for?

Hmmm...it isn't quite as easy as that. There is a lot that must be considered when choosing a bike size. Assuming the bikes you're considering have parallel top tubes (as opposed to the newer sloping top tubes), the general rule of thumb is that when you stand straddling the bike, feet flat on the ground, you should have about 1" or so clearance between the top tube and your crotch. Mixte, women's, MTB's, and road frames with sloping top tubes require different measuring procedures (usually involving what is called "virtual top tube measurements"). But....there is also the matter of frame geometry. Just because you may have the requisite ~1" clearance doesn't necessarily mean the frame will be a good fit.
Probably the best thing you can do -if the seller is willing- is to take the bike(s) to your trusted LBS and have them assess the bike with you on it. They'll be able to tell you if the bike can be adjusted to fit you properly, of if it will be an exercise in frustration. If the seller is not willing to help you, try the toptube/crotch fit, check to see if you can adjust the saddle and post, stem and bars to an approximate fit, and decide from there. I'm sure others can add more info here as well.
FWIW, I'm 5'11" with a 35" inseam. I ride anywhere from 21" old skool MTB frames to 23.5" old skool road frames. Good luck.

~Sherry.