urgent! is it legal to block

a personal check? i was buying a house. the seller accepted the final offer 7 days ago. but now i canceled this deal. when i gave out the first offer, (this is my first time to buy a house), my agent said that i had right to cancel within 17 days after acceptance. i believed what she said. but now after faxing the cancellation form to the seller and escrow, the seller was very angry. they posted the house back to the market, without sign the cancellation. they don't wanna return my deposit (it's a lot of money). i don't know what i should do now. i even could not reach my agent. i tried to call her cell phone or office number, but only her secretary answered my phone and told me that she could not reach my agent too and that my agent was out of town. i could not believe it, but nothing could i do. so i want to ask if i can block that check of deposit? is this legal? anyone knows the law or anyone had the similar experience? pls give me advice soon! thanks in advance. waiting online!