I have seen that 'Slots' seem to have a meaningless effect for me, so I don't care or have no opinion about them, but.. as for time for mats to expire on merchant, well, seems 6 days is a bit to quick, so i would not be opposed to something like maybe 2 weeks, or maybe 1 month at the most, but never would I agree to 6 months. Imagine wading through 12 million crafted mats/items/ etc. that have been put up on the merchant in the last 6 months? Forget it! It dictates that it simply will not sell, so let it be lost.
If you want it up longer (for storage, admit it!), then pay for it! 1 month is longer than enough to tell if the item will sell.
If you want it up longer (for storage, admit it!), then pay for it! 1 month is longer than enough to tell if the item will sell.