That is definitely not what will occur.
Example: Party finds a decanter of endless waters. The higher the trade value of the item, the more likely said party will trade it in for gold which can be applied to a specific item tailored to them.
Seriously, how can you NOT see the 50% rule encouraging people to trade "junk" magic items for gold.
Like I said...they have every reason to take a possibly useful item
until they find the need for cash for something they want more. It's the same in 4E, just that you're giving them less money for selling it. This means, obviously, it's more painful to sell things off and the party might hesitate a bit more, but I don't see why it's necessary. If you really want the party to hold onto items as hungrily as possible, why not only offer 5% of market price for them? Then they'd almost never sell things to stores!
Now, 4E also explicitly says the DM should take "wish lists" from players of items they'd like, to place in treasure as a special reward. So this is ultimately just shifting things around. Instead of saving up and buying that shiny new sword, it miraculously ends up in the treasure pile. I'm not against this, in fact while I don't ask for a wish list, I do try to purposely put in treasure once in a while blatantly useful to the characters. Often it will make sense, like the rogue getting a nice fleet warrior's array item off a fallen scout enemy, for example. I don't need to make items sell for 20% to make useful items "special." Effectively getting it for 50% is still a heck of a deal.
The party may get more for the decanter in 3E, but it's still nice to have. Maybe it's just the people I've played with have never been that absolutely devoted to getting one specific item (obviously, occasional exceptions, like most wizards wanting a blessed book ASAP). They tend to never spend so much that they have almost no gold left, and they don't mind waiting a bit longer to buy a particular item if something less ideal but cool turns up in a treasure pile. Generally, the attitude I most often see is along the lines of, "Well..I wasn't going to get [item] until maybe later...but if I ever get it at all, might as well be now, rather than pay full price later. I can just sell it next session if I change my mind."
And I still don't see how 20% is more realistic when the items work just as well as when they were new. IF the armor is cracked and cut, then sure. It's not worth as much. But if it's +1 full plate and giving a +9 armor bonus...it's clearly not that damaged. think about it from a wizard's perspective: He could take this wand at 50% market price and sell it for full or...spend that exact same amount plus experience points plus time...to sell it for full! What's the better deal here? You want to simulate limited storage space or on hand cash? For the former, the DM can always say no. "That mage has no desire to buy that rapier from you, cause this is Dwarf Land, and Dwarves don't buy rapiers." For the latter...the DMG has rules for this already.
