Only the BUYERS feeling of wanting it!

On regular and commonn hats, not that much. On rarer / wanted hats, quite more. It can range from 1 scrap to 2 refined, whatever the buyer is comfortable to pay more. Though there are some scurbs who sell uncraftable level 100 hats for default price+1 key or more. You have to try it out to see which price sells and which not.

Its entirely arbitrary.

Levels are gambling, craft numbers are worse because people ask far more for craft numbers when most won't sell at all above normal price. Paint depends entirely on demand of the base item and how good it looks. The math formulas on tf are just a bit of nonsense on all these issues. Its like anything else, you get what you can get.

I've been offered enough craft number items to see they are mostly a scam, you see people just say their craft hat is craft number 50 or whatever, so its worth 6 keys! then they ask to buy something from you...its just very scammy. I'm sure a few craft numbers get sold for keys but the rest, are just used to try to con someone into giving them key items for a craft hat.

Paint as long as the price is less than doing it yourself is reasonable. but if you need to sell quick or the item is kinda lousy/color kinda lousy, you might have to eat the cost.

Its like putting a long distance kills or robots destroyed part on some weapons, you may have cared, but most people won't, so you may not be able to charge for it.