With 1Ghz T-Birds, the most important thing is what stepping it is. You can easily read the stepping by looking at the second line of code etched into the core of the CPU.
Example:
First line: A1000AMT3C (B meaning 100Mhz, C meaning 133Mhz)
Second Line: AXIAR0123GPMW ( the first 4 or 5 letters are the stepping, the 4 #'s are the date of manufacture: 01=2001, 23=week 23)
Third Line: Y6464880170 (Y is better than 9 for OC potential)
If the used T-Bird you're considering has a C at the end of the first line, AXIA or AXIAR on the second line, then $55 is a deal. And if the third line begins with a Y, jump on it, as I have one of those chips that runs rock stable at 1.45Ghz.
I also have a 1Ghz with this stepping: B, AJFA w/9 (one of the first T-Birds made), and it won't even do 1.1Ghz. Very poor overclocker.