Can I back out of an accepted offer?
Only through a condition, before its deadline. Otherwise you are in breach: the seller keeps your deposit and can sue for their loss on resale, which Ontario courts have awarded well beyond the deposit amount.
If a condition is still live, you notify the seller within the deadline and the agreement ends, with the deposit returned once both parties sign a mutual release.
If the agreement is unconditional, you are obliged to complete. Walking away is a breach, and the seller's remedies are real: they keep the deposit, they resell, and they can claim the difference plus carrying costs and legal fees. In the falling market of recent years, Ontario courts have awarded sellers hundreds of thousands of dollars against buyers who did not close. Some buyers have been pursued into bankruptcy.
So the deposit is the floor of your exposure, not the ceiling. If you are having doubts before signing, do not sign. If you are having doubts after signing and you still have a condition, talk to your lawyer today rather than after the deadline.
Next questions
Is there a cooling-off period after I sign an offer in Ontario?
Not on a resale purchase. Once both parties sign, the agreement is binding and your only exits are the conditions you wrote into it. New construction from a builder is different and does carry a statutory rescission period.
Ontario
What happens if my financing falls through after I have an accepted offer?
If you kept a financing condition and it fails within the deadline, you walk away and your deposit is returned. If you waived it, you must close anyway or breach the agreement, losing the deposit and exposing yourself to a claim for the seller's loss.
Ontario
How big should my deposit be, and when is it due?
Big enough that the term reads as serious, and the offer tool in this app puts that at the middle of the three-step band it publishes: five percent of the price. That band is this app's own scale, not a measurement of anybody's market. It is credited against your down payment rather than added to your costs.
Market practice