Do I pay US taxes if I live in Canada?

You will generally have withholdings, though the best person to answer this or any tax questions would be an accountant or CPA. "What percentage will I pay an agent to sell my unit" is a completely different topic. Commission is completely negotiable between you and the agent you hire.

You can use your 401K to buy a home. Some employers allow you to take out a loan from their 401(k) for various purposes, including buying a home. Keep in mind the loan must typically be repaid within a specified period, often five years, and failure to repay it may result in penalties and taxes. Before tapping into your 401K understand the terms and consequences of a 401(k) loan before proceeding. An alternative might be a down payment assistance grant if you qualify.