Traditionally people would buy a property first and put a condition on the sale that it is conditional on them selling their property. But some real estate agents encourage buyers to not put conditions on properties these days, which imo is risky for buyers unless they do a lot of due dilliegence before hand, which is expensive if you miss out anyway.
If I were selling a house and a buyer had a condition on their offer subject to selling to their own house I would simply ignore it a turn my attention to the next offer. This isn't the UK, we do not entertain the idea of "chains" in the sale of houses.
If you want to buy my house and still have another house to sell, then you need to get bridging finance.