By looking at comps on the market that are similarly in size, upgrades or not, basement/ no basement, garage, backyard size, etc it can help you price out the property you're scoping out to re-sell or rent.

If I paid $50 for a game and beat it in two days and took it back to the store, they would purchase it back for around $3 to $5… then resell it for $45!

Hard to resell.

I find him projects, manage them and resell them.

Here where I live, not a good idea to invest in condo's to resell, but if looking to hold, even with the HOA's, there are some good foreclosures.

Also can using a double closing/ transactional funding at the closing allow me to legally market the property due to the intent of purchasing it for myself only for a bit and then reselling the property I own? 

This will save you money if you are reselling in the next few years.

I'm a real estate investor who buys beat up homes, fixes them and resells them.

You are the lender so you can foreclose on the property if the buyer can't pay the note, and then re-sell it to someone new if that ever occurs.  

Of course, if you want to achieve $5k-15k EXTRA equity from day one, do what Wholesalers do - except KEEP the properties instead of re-selling them!