If folks could make money running a shop where they consistently sold items below comfortable margins, there would be a shop on every corner. It's like natural selection; shop owners that are too stupid to stand their ground on profit margin simply go out of business.

If you don't buy it, someone else will. Don't assume you are the last bastion of hope for the shop to get rid of that part that you must have at a 30% discount. Your money is not greener than the next guy's.