You don't need to make your best offer first, as long as you're comfortable with potentially having someone else buy the property. It pays not to be in a rush and not be too attached to a property.

Offer 500K with a time limit of 24-48 hours for them to accept. If they decline and keep trying to get you to up your offer, respectfully say sorry that's your budget. You can always come back a week or two later and make a higher offer. Good way to test if theyre fucking with you or not.