There are a few possilities here.
Either the table was always messed up, you just weren't picky enough to notice (the buyer REALLY should have played a game on it first...there's no guarantee the seller has high standards),
It got messed up when you moved it (you'd probably remember dropping it, wedging it in an awkward spot, etc),
It got messed up when THEY moved it/used it/did something to it (in which case it's their fault),
Or it's not messed up and they're making it up.
The only case in which this would be YOUR fault is if it was damaged when you moved it, and that sounds unlikely.