Dasius has been recommending people to simply finish the trade, get the money, then save and reload - this will clear their temporary spawn data
I recommend only selling them one item, and then sending them away.
Make sure they only accept either the weapon, or the shield, and then send them away. - They aren't familiar with "returning" items- and will always pay for what they've taken before leaving - Likely set in place because someone may not finish making the second item before time is up- So the customer still has to pay when they walk off.
Most of the time - a shield will be the choice, as it is 5 ingots worth of value.
But sometimes you may discern that it is more profitable to do the weapon - In cases where you have 1-3 next tier ingots (Only applies if you have an Alloy Hammer or next tier hammer already made and are just waiting to make a sale in the next tier.)