I don't really think it was selfish of you. If you were the first one to have it in your hands with a certain amount that you intended to buy, then it was selfish of her to assume that you didn't need what you needed.
I don't think I'd spend too much time thinking about it or feeling guilty. I'm glad you didn't let the cutting clerk 'use you' that way. I would have felt totally bummed if I needed/wanted a certain amount and someone basically took it out of arms, took what they wanted and I was left short--when I had done the searching for and holding the bolt.
The other lady could have the store clerk check and see if other stores in the area had any.
Any sale is "first come, first served". Once I had a deluxe Scrabble game put aside at a yard sale. Unfortunately I put it down and someone else didn't realize I had meant it to be with my stuff--(I had put it close to me in my 'pile', but turned my back). I didn't see that it was gone til too late and then I didn't feel like making a stink, but to this day I feel like I had chosen it, put it down by me, but didn't notice, because you know how yard sales are.....
So good for you!