There's no security deposits and payments are made weekly.

I then do the REAL negotiating and get the seller to commit to a contract and give them a deposit. 

The proper way would be to establish 401k plan first, fund it, then look for a property, sign the contract in the name of the 401k, pay earnest money deposit from the 401k account, arrange financing if needed and close on the property taking the title in the name of the 401k. 

She called me and wanted to walk from the deal and wanted her full deposit back.

Can I use home depot's receipt (material) as claimed to deduct on the security deposit disposition?

I sent a letter claiming intent on their security deposit Dec 1 to the last known address according to Florida landlord-tenant law.

Make sure you got her deposit transferred to you and get the prorated rent. 

I make my deposits just under the rent (helps with avoiding confusing the deposit with the last months rent) and if they want me to hold it, they pay that full amount.I really recommend you spend some time reading in the Rental Property and Landlording issues forum.

Changes of late fees, grace period, due dates, rental amounts, deposits and other material changes can only be made once the original term ends or is lawfully terminated.

Think of all the things that could go wrong and see if the lease addresses them: unauthorized pets or tenants, early termination, security deposit, lease violations, late rent, eviction, lawn maintenance, parking, etc.5.