So what is our obligation to buy from our local store?

The way you phrased your question shows the answer you'd prefer to get, but as most here have pointed out the amount of help you received from the store's staff and from directly trying on what it carries in stock is worth something to you. As you learn to appreciate that value, you will understand the need to reciprocate it, by becoming a valued customer.

There's no hard and fast rule about what you are "obliged" to do in return for their aid and assistance. Just recognize it for what it is and do what your heart tells you is best.