I'm sure it would be reasonable for you to ask the store person to bring out the PSP and let you see the firmware.  After all, you only need to pay for what you expect (consumer protection laws).  If you expected FW6.20 and it comes with FW6.37, you can complain that you didn't get what you expected, so long as you've made it clear to the store of your expectation.

If the store person simply refuses to let you see no matter what (shouldn't be the case), then just tell them you'll find another store that will (and do that if they still refuse).