3 batteries at 7.7 volts !!! That is a 10% state of charge (90% discharged). I can only assume they had been sitting around for a while and some permanent damage had already set in. You did very well running away from this "deal" (or should I say "steal")
The batteries that you got at 50.3 volts, average about 86% state of charge ... you will surely be pleased with those
You demonstrate a very good lesson for others to learn when buying "new" batteries ...
bring a voltmeter with you and be picky about what you will accept.
What is the date stamp on your new US Batteries?