Do extra parts add to set's resell value?
Generally, I think it doesn't add much value. But it depends.
Some sets, like US version of 8284, have extra parts crucial for alternative model (aka B-model). So unless you include them, you are selling incomplete set. The inclusion of all parts to build all models is part of &quot;complete&quot; condition description on Bricklink. Extra parts excluded though.
Then again, new DOTS sets, which have random printed tiles, but the number of included items is the same. So if you are looking for particular print, it would be nice to have extra.
Sure, there's a ton of sets with quite generic extra pieces, that you may already have a ton of. And buyer may as well already have a lot of them. However, buying a used set with box, instructions, stickers/sticker sheet as well as extra parts is always nice, especially if it is older set.