Action should be adjustable - makes sure you can do so. Seems like the seller didn't look at it as close as you did. Since he's cooperative, figure out what it will take to make it right and negotiate a partial refund. If you're not comfortable soldering pots, factor in some tech time.
The alternative is to return it, which is likely not desirable for either party.
The alternative is to return it, which is likely not desirable for either party.