A person who entices (or enables) another knitter/crocheter into buying yarn he or she doesn't need for a project, only to add to their yarn stash. Usually in reference to a pricey/luxury/particularly pretty or desirable yarn. A yarn pusher.
"Mom, this yarn is so pretty! Buy it, I won't tell Dad!"
"Lydia, you should buy another skein just in case you run out!"
"Omg you are a total yarnabler."