Making a contract subject to sale of properties is to broad. There needs to be a final date - what happens if it takes 2 years to sell the final house? Do you still want to be bound by it? What about if during the sale of the 2nd property you get a better offer from someone else??
I think the agent gave fair advice. Unless it's all properly drafted it's hopeless. Just like "subject to finance" clauses. It's best for the buyer if they state by which bank and what time frame otherwise the seller can take legal action if they can prove that some finance company can finance this loan for the buyer (probable ok the loan at 8% for example)
