Assuming the set is complete, would the value for example be $5 or 10% less than market value if the instructions are missing? Is there a general rule you'd look for when buying?
Thanks!
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Sign up to join this communityIf it's a newer set, missing instructions will usually just knock a few dollars off the price, even for larger sets. There are usually enough adult fans buying sets for parts that it's easy to pickup cheap instructions on Bricklink. For example, instructions for the new Millennium Falcon can be purchased for less than a dollar in new condition on Bricklink.
However, instructions and boxes for rarer old sets often vastly impact the price. For example, instructions for the classic Airport Shuttle (6399) are currently going for $40 dollars used and $100 new.
Basically, for newer sets the difference in price should only be a few dollars. For older sets where instructions may have been lost or damaged, the difference can be much greater.
The value of the set is indeed greater if it comes with instructions (and even greater when the set comes with a box, but the increase in value is not as big as with the instructions). Especially with older, desired sets, a lack of instructions is sometimes a sign of a BrickLinked set (a set built from a collection of pieces bought from BrickLink stores), which are less desirable than sets built from the original pieces.
So the order is: NISB (New In Sealed Box) > Used, Complete with Box and Instructions > Used, Complete with Instructions > Used, Complete > Used, 99% complete (or any variations thereof).
There is no real guideline for the price difference as far as I can tell - sometimes an NISB/MISB (Mint in Sealed Box) set sells for about as much as a built one, sometimes the price difference can be quite big.