WIW - fresh 383 short block- I need a ‘I can’t turn this down price’. ie: what’s a good deal?

Im always skeptical of rebuilds that I cant hear run, buying from a stranger. You could base a offer on what the parts are worth to you
If you dont need this engine, the value to you is low. Figure the value of parts and machine work and 25 % of that would be my offer, more if I needed the pistons, crank etc