Every player plays their part.
The Agent wants to list and sell fast.
There is no perfect CMA this is all based on current homes sold. The Agent averaged out a range of homes that came close to what you have to sell. Although such a bad sample (1000 sqft off the mark) can really change the numbers on the CMA. If the Agent didn't visit the home to access any repairs needed, the Cash Buyer's CMA might be more accurate, but be warned this is a Cash Buyer.
The Traditional Home Buyer wants to buy low. It doesn't matter the cost of the home or the CMA, whatever a person will get approved for by the bank, that is what they can spend, period.
The Cash Buyer wants to buy low to live in, rent out, hold and flip years later, flip right away for a profit, or a combination of those but one thing is clear if this is a Cash Buyer they need to make a profit so they will buy at around 70% of the After Repair Value. If the home is in perfect condition no repairs needed, take the market value for the home and deduct 30% percent, that is what he Cash Buyer will pay. If repairs are needed, get an estimate of all repairs needed and deduct that further. So 70% of market value - repair cost - closing costs (depends on if you are paying this or they are) and you get what a Cash Buyer will pay.
The difference between $755 and your appraised value of $780 is $25,000, why not meet the Cash Buyer in the middle? This must be an amazing house with nice features for a cash buyer to pay will above the norm of 70% of market value ($550,000 in this example)
Note: If you list the property and you know the CMA already $780, how many people you know will pay retail for any house in the current market? Not many. Now your location is key, the area might be coming up (values are rising) or going down. Ask this to the person that gave you the CMA for $780. If its going down, I'd take $765. If its going up, I'd take $775. If you need to sell now and by selling at $755-$775 you still pocket an acceptable sum on the sale, then take it!
Look at how motivated you are to sell, your need to sell, the trend of home values in your area, this is key.