Take the used 53 cm for a ride, just see how it feels. (well, first be sure you can stand over it safely)

Since you've already bought it, think of it in terms of trade-in value. If you absolutely love it when you try riding it, you have some decisions to make (is it worth fitting with the various things Danielle told you about? Is it so close to perfect that you feel it needs very little extra investment?)

If you take it for a ride and don't like it (or if you can't stand over it without pain) then look at it as your trade-in for a smaller bike. Or see if the seller will take it back. (if it's a fabulous deal, see what your LBS offers for trade-in. They might offer the same you paid. Makes life a little simpler.)

I'm 5'8" and ride anything from a 56cm to a 52cm, depending on brand. Your best knowlege comes from some experimenting with your favorite brand. Ride a few sizes and see how you feel on each one before you make any decisions.

Feel the bike love!