Deposit a lot of money at a bank you will do business with in Illinois.  

If you are self employed you can also look at bank statement loans assuming you have a business with high deposits.

An STR will look every bit as good as it did when it started - because it has to.Not only that but it is much easier for property managers to get repairs reimbursed when a guest damages something as an STR as opposed to when a long term renter moves out and outside of the security deposit, good luck. 

Have them direct deposit into a separate rental account if you can.

Also check how long you have to notify the tenant that they won't be getting their full deposit back and how long you have to return the balance.

For instance here is a snippet I saw on crexi:Non-ContingentEarnest Money Deposit: Greater of $20,000 or 10% of the Purchase PriceMarketing Fee: 5% of the Winning Bid Amount, $20,000 minimum$10,000 Participation Deposit

Im tired of letting tenants bail out with unpaid rent and damages that exceed well past the deposit. 

The rehab funds are for completed work only so your parents would need to front the contractor like 15k let's say to get started and then they get their deposit back at the end of the project because they added 15k to the rehab funds with their deposit so 15k will be left over in the escrow account when job is done. 

The Brokerage advises agents NOT to handle deposits, but have buyers send it directly to the closing company (e.g., closing attorney or seller attorney).  

Many tax sales require deposit up front and payment day of sale or soon thereafter.