In my opinion....Selling engine blocks for good money is difficult unless you're willing to spend some money up front or at least stand behind the sale.
Here's why....You can't confirm the bore size, You can't confirm what it might take to clean up the bores and you can't confirm whether or not the block might be cracked.
If you pay now to have it hot tanked and magnafluxed, and properly measured, honed or even bored the buyer will know what he's getting exactly.
You will have a better shot getting bigger dollars for a fresh block, then for a mystery block.
If I take "your word" that it's good and I pay you $800 for it, turns out to be cracked or .060 over and useless.....will you give me my money back? Where would you like me to drop it ? literally
If it were me, block and crank would be $300 as a mystery block.
I had a mystery 383 in the last Barracuda I parted out....I had no idea what was inside, didn't have time to check, I just wanted rid of it fast. Listed it on KiJiJi for $150...Guy insisted on pulling a head off for inspection while it was still in the car...turned out to be a freshly rebuilt motor with zero miles...He gladly paid me $150...