The math behind these programs works because the guaranteed price is set low enough that the brokerage rarely has to actually buy the house; most properties sell to a genuine buyer well before the guarantee ever triggers, at a price the seller would not have accepted upfront.
Read the fine print on timing and condition requirements closely. Many of these offers require a minimum listing period, often 60 to 90 days, a specific list price and a set number of showings before the guarantee applies, and missing any one of those conditions can void it.
A guarantee is only worth taking if the number itself is one you would accept anyway. Get your hand-reviewed valuation from Joel Dyck.