How do you choose?

I am currently looking for a new amp and noticed a few that were sold at a higher price than the lower priced amp (about 500)with the same rating. I thought this was rather strange. when your picking up a used amp what should you/ do you look for?
depends on several factors - what preamp you are using, what speakers you are using, what size you room is, what type of music you listen to and how loud..etc, etc, etc!
I would not really worry about the ratings until you have a plan as to what to buy.