It doesn't matter what the asking price is. And it doesn't matter whether the buyer (or, presumably you mean the seller) has an agent. That's as meaningless as the number of letters in the condo's name.
The primary thing that matters is what the condo is worth. If it's worth only $265,000, then you're overpaying at $270,000. If it's worth $350,000, then offer less if you want, but it's a great value at $310,000. So, forget asking price. Focus on its true value. And that figure is the ceiling price you should pay. Not a penny more.
You also have to consider what you've been prequalified for. Further, regardless of preapproval or prequalification, your comfort level may be lower. A lender may say they'll fund you up to $300,000, but maybe you know that you really wouldn't like to spend more than $285,000.
Now you take "true value" and your "comfort level number." The LOWER of those two becomes your ceiling price.
Now you can offer that amount, or some number lower than that.
That's how you determine a reasonable number.
Hope that helps.