The rationalizations in this thread are utterly ridiculous! SteveC is totally right: the 1/5 sale price is for game balance, and has nothing to with anything else. Even the 1/2 from 3rd was on the low side.
Wear and tear has <i>never</i> been an issue with D&D magic items - these are things you pull out of the belly of a displacer beast, find by sifting the sludge at the bottom of the keep's latrine, or pick up off a corpse that's been moldering at the bottom of a pit trap for a few decades, and find they're as good as new!
Secondly, magic items are extremely high value, low weight goods. Security is an issue, but aside from that, dealing in them is <i>ridiculously</i> easy money, especially in a place like Sigil, Waterdeep, or even Cauldron. Keep in mind in 3rd, 1 sp was a day's wage for an average joe. 100% returns aren't enough, now they need 400%?
I'll admit the economics of D&D never made a lick of sense; I'll also admit the PoL idea would make the magic item market a lot less liquid. However, it's obvious the 20% sale price is about game balance and nothing else. That's fine, 4e is a more "gamey" game, and they want to get away from looting everything of value. These are fine, defensible choices, even if they might not be the ones I would make. But please don't tell me it's about realism.