Perhaps, but some people trade with cash to get their money out of Steam

Are you able to withdraw the money onto your credit card? If not, i think its fairly stupid.
a) You lose the item from your inventory
b) Steam money only goes so far, this may work with low tier items, maybe upwards to 150. But above that would be fairly pointless unless you want to sell an unusual for a lot of games.

Just my 2cents but its just steam wanting more money..

Well, maybe not normal people, but game developers and users that have got tf2 workshop items of theirs ingame should be able to.

might be interesting to see how this effects the unusual market in the long term (long as it doesnt get totally messed up)

the only unfortunate thing is most people dont use or havent heard about the market

Steam takes 15% from the seller. Paypal doesn't. It's economically inefficient to use, as the goods are priced higher than they are on the paypal market, and the seller gets paid in a currency that can only be used to buy games or more overpriced shit off the market.