@Brad T. great questions for a new wholesaler so you know what to expect going into the business vs get pissed later and mad at someone else for your lack of experience. So I appreciate great questions. :)
I believe it's all been explained above already, but always free to clarify anything as I've ran a wholesale company in Atlanta for years, then mentored across the US from 2018-2022 (stopped end of last year to travel and enjoy life every day).
So IF you choose to double close, then the option of paying for your closing costs on your purchase (the AB transaction) is optional but NORMAL. IF in your contract with the seller you make them pay, you can but normally you as the buyer would pay for your set of closing costs. Depending on the state, usually the taxes are a wash as it's prorated, then when you sell same day it's prorated and again a wash. But your attorney or title fees will be on your if you as the buyer pay for them on your AB side.
Then if you double close, when you sell, your end buyer would then be paying for their closing costs UNLESS you already had it in the contract with the buyer that they also pay for your AB closing costs (which I find extremely annoying and rude personally, and if you wanted to do that just build it in the price vs telling them they're paying for your choice to double close when you could have assigned the deal).
Nice segway, so I'll run with it. If you ASSIGN the contract and are transparent with the seller and your end buyer, you'll never need to double close. There will only be an AB closing, then your asssignment fee will just be a line item on the HUD/Settlement statement and your seller will now be direct with your end buyer and one set of closing costs that your end buyer will be on the hook for usually.
If you don't have to double close, then don't. Save everyone time and money. But if you choose to double close for various reasons, keep seller separate from buyer, keep your fee private, someone refuses to sign an assignment contract, the closing team doesn't approve (stupid) or some other reason, then yes, double close.
Depending on the state laws or the closing company will also help make your decision on double closing and your additional costs. Most states you can use "pass through funding" meaning your end buyers funds in escrow can be used to fund your AB purchase. However, some of what I mentioned above won't allow you to use pass through funding and would require a lender (like me) to fund your AB side if you didn't have your own capital in the bank to fund your purchase before they'll close the 2nd transaction (BC) side.
With all that said, try to be transparent with everyone and assign. but if you need to double close as backup make sure to calculate that cost ahead of time and still keep a good spread so you'll make a profit at the end of the day. Then if you're in a state or dealing with a closing company that requires you to fund your AB side, then hit me up since you'll need Transactional Lending (funding) and I'd love to help you or anyone else out. :) :)