I think the above is a creative idea. I like it.
Selling pins at cost or close to cost is always a loss, but is it kind (and I love love love kindness.) The gain is zero for my efforts (time, mileage, etc.) if I kindly sell a pin at cost (and as stated, I do like to be kind in this hobby, but what I really want to do is trade with kindness.) If I trade a pin, my efforts pay off in an equivalent fashion. In a trade, the pin I receive also has someone else's effort behind it. Pin trading is basically trading your effort for another person's effort. Your idea above allows you to kindly offer the set at cost, but also gain something for your efforts. Nice.
And congrats on completing the set!