I think making items tradeable would have major positive effects.
Firstly, it would remove the scamming related to having to gift items. The argument that scammers will always exist is irrelevant, it doesn't mean their lives should be made so easy that they have to do next to nothing to scam. Being careful and taking screenshots doesn't prevent you from being scammed, it helps after you have been scammed.
Secondly, I'm pretty sure it would encourage more people to buy IM, since you might be able to get a higher price from your items if you are willing to wait for the market to buy them, and selling your IM would require less effort. On the other hand some people who might now buy IM because they can't get the items easily with gold might then stop buying IM, though I doubt there are many of such people.
Thirdly, it would be less p2w, because items would be easier to gain with out using your bank account. No longer would you have to look for an IM seller, just type the name of your item in the market search. Easier access means less pay to win, no matter the case. I don't understand why people bring eu into this, that server was pay to win because of much higher prices of the shop items, obviously. Insurances being tradeable already is very good, but some other items I would at least like to be able to easily buy are:
Randomizers, required for top notch gear and you need a lot of them. Being able to easily buy these would reduce p2w.
Megaphones, on eu people would only buy 1 megaphone from the market when they needed one, making it easy for even people with less gold to use them often. Very useful for finding and forming parties.
Beans, would just make lvling faster if these were easy to buy, and people could buy them from the market for their run and split the cost (positive for the community). I've heard many people complain about the rate of lvling in DN, and although I don't see it as a problem, those who do quit early, reducing the amount of players we have. Some people would start always wanting a beaner in their party and standing in front of the entrance declaring so (negative for the community), but if you dont care about that why not make your own, beanless party.
Change of this caliber would also have its downsides.
Economy wouldn't react well to making so many new and useful items be marketable, that is true and that's why I think only the few I mentioned would be enough and a change like that wouldn't destroy the economy.
If untradeable IM items are truly an effective way of stopping duping IM all for it, however those 3 items I mentioned would be cool to have in the market anyway.
How it is now works, but is inconvenient and scam heavy. Maybe once the development transfer is complete we could see a system similar to the current one, kind of a charge mail with gifting or something like that.
What about just making the items able to be charge mailed? That wouldn't change much at all other than remove the scam you can't prevent in anyway except by buying from a known and trusted seller, and who's to say this trusted person hasn't suddenly turned mad at scam you for all of your gold.