Good advice here, all around. But I would suggest going to see the bike anyway.
It probably is NOT a 19" frame but maybe there's some peculiarity about this frame and it is. As you are new to this: frames are generally measured up the seat tube, from the center of the crank up to *either* the middle of the top tube, or the top of the top tube. Whether it's 'center to center' or 'center to top' can obviously make a difference. In this case though, we are looking at the head tube for an estimate. Generally on a steel road bike, on a 19" (or 49cm) frame, there is very little gap between the top tube and bottom tube where they join the head tube. As you can see, on this one there's a couple of fingers' gap. The next size up from 19"/49cm is usually 52cm, and this looks like even more than that. BUT maybe there is something in this bike's geometry that makes it different. (The seller seems to know SOMETHING about bikes so it's surprising he'd get the size so wrong.)
I will say that given the special hook the bike is hanging from, the shiny paint, the accessories on it and the updating, with a newish (if ugly) saddle, etc, the current owner has evidently taken some care and spent some money on it, so it seems likely that it is well maintained and will really be ready to ride. That's unusual.
Even if chances are this bike is too big for you, go see it anyway, you'll learn something.
BTW 19 1/2" (more accurately) or 49cm is probably right for you; I am 5' 6" with fairly stubby legs and I am best on a 52cm.
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I never think I have hit hard, unless it rebounds.
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