Absolutely, it's a fine question to ask! I mean, I assume you're not going to say, "hey, give me a skein of your yarn or I'll flatten all your tires."
But a nice, "I'm in desperate need of 1 more skein of TruBoo and everyone's sold out, would you be interested in selling me one?" is very appropriate, and sometimes even appreciated. You never know what someone is trying to get rid of!