5' 1.5", that 43 should work
with a 28 inseam, you could go bigger...
my guess is that the 5' 1" woman selling the trek prolly has a shorter inseam...
$250 for that Trek is an OK price, even if you need to get new chain, tires, cables and bar tape, the overall price is good.
but always best to have someone, who knows, inspect the bike for what it might need.
you might be able to get away with a 49/50, bu that would likely be the top borderline...