Out of your Profit, you deduct debt service. 

You can contribute up to $5,500, but may not be able to take a deduction for doing so. 

@Paul K.I would advise on the following1) NOLO's "every landlord's tax deduction guide2) BP's The Book on Tax Strategies for the Savvy Real Estate Investor3) IRS Publications on depreciation, rental properties etc

I also would not have told him you were selling it because if you do deduct things from his security deposit he will complain that, "you were going to improve that, or fix that anyway to get it to sell."

@Parmer Cherry, Yes you can donate items to qualified charity and possibly get a deduction. 

Since pretty much all the other expenses are tax deductible? 

Once you have that number then you can deduct your profit and costs to determine your offer.  

I would look at the market around my area and see how much real estate goes for my area. check if my online investing can cover the cost of buying and rehabbing a property after selling those investing deducting tax.

So, you will benefit from deducting depreciation for 3 years, but then you will have to return this temporary benefit when you sell - essentially a wash.And, as @Jon Holdman said, the deadline is firm, cannot miss it even by one day.And - very important - the rule assumes you do NOT move back.

We are deducting an additional $265.18 from your security deposit to cover the damages to the door."