That depends on when the seller says the auction ends.
If the time is relative ("24 hours after last bid"), then you can look at the time that the last bid was posted, which will be in your local time, and use that with your own clock to determine the end time.
If it says that it ends at 2 PM PDT (the user's time) and you're on EDT, then it'll be easier for you to use the "User's Time" that I explained above under (c).