Buying first without a confirmed sale means a lender has to qualify you while you are still carrying your current mortgage, which is a harder approval and, if it does work, leaves you paying two sets of mortgage and utility costs until the first home closes.
That carrying cost adds up quickly. Two mortgage payments for even a few months, on top of legal fees of $800 to $1,500 on each transaction, is a real expense most buyers underestimate when they plan to buy before selling.
For most sellers, avoiding that double carrying cost outweighs the inconvenience of a short possession gap. Get your hand-reviewed valuation from Joel Dyck.