Proper Marketplace Etiquette

A couple quick questions regarding marketplace etiquette, that I figured was best answered directly by an elder.

When it comes to crafting bags, if someone posts a request for a 12 slot cotton bag, but you're an armorsmith. Can you fill the order with a 12 slot steel box? I know sometimes people placing the orders (myself included) say that it doesn't matter which bag type they receive, however, it's not something everyone includes. Would it be better to just hold out, until someone posts a request for a bag type you can make?

And, number two, Upon advancing beyond X slot bags, if you requested them from the Marketplace, is it reasonable to fill other orders with those bags? Seems like a grey area, since you did not craft the bag itself. But, otherwise.. it might not get used. I know it's taboo to sell it, but I'd like someone to get use out of a 12 slot bag after I've moved on to 15+'s. 

Thank you for your time, and I hope you have a great evening. :)

Yes, you can fill orders with items you buy as well. Note that you wont get all of your points back. I've set this thing up to lose points when you try to fill an order with the thing you just bought from the MP.

Send the person a mail and ask if they will take a bag instead of a box etc. Most people dont care.