Well, here's an idea for people who don't like the 20% markdown who still want to keep balanced with the expected cash per level chart in the DMG.
For simplicity, let's just say a party of 1st level PCs get 1000g on the way to level 2. A PC finds an upgrade to his magical sword so he wants to sell his old one(worth 200 gold base). Through several days of negotiating and haggling in a large city, he manages to get 50%(100 gold) for it instead of 20%(40 gold).
Now, if you just let PCs do this without balaning it somehow, they'd end up with more cash than the game as designed expects. This becomes more exaggerated the more you let them sell items for and the longer this goes on in the campaign.
So, all you do is take the extra 60 gold the PC made from selling the sword(100 sold for - 40 its expect to be sold for) and subtract it from the 1000 gold the group was supposed to get on their way to the next level. They'll get less treasure from monsters this level, but they'll still end up even at the end.
This way, the PCs get the satisfaction of getting a bit more money for their old gear and DMs with issues with the 20% can have what they consider "realistic" sales without breaking the game's money balance. Unless you're really unlucky and the party is keeping track of exactly how much gold you have given them and is cross-referencing the DMG, the players probably won't know the difference.
This system of deducting "extra" rewards from the "level gold pool" also gives you balanced ways of handling situations like the following:
A) The players climb a mountain to find a rare herb that grows there to use in a ritual. After a skill check or whatever to find it, the players get the rare herb. You decide it's equivalent to 100 gold of ritual components, subtract 100 gold from the group's level gold pool and carry on.
B) The players kill a dragon, get its treasure, then get the clever idea that they can sell its scales/teeth/eyes/whatever for potions/elixers/herbal cures/rituals/aphrodesiacs/whatever. You like the idea and so decide what they harvest is worth 75 gold. Subtract 75 gold from the group's level gold pool and carry on.
The PCs feel like their ingenuity is being rewarded and everything(hopefully) stays balanced.
This is the system I think I'm going to use. I might use their pre-made parcels from time to time, especially if I'm winging it, but for the most part I'll just keep their treasure total written down somewhere and any time they get something of value, I'll just knock its value off from their total.