Certain magic items will sell in the open market for more than they are listed for in the DMG and UA. Specifically:
Armor:
Non-magical field plate sells for 5000 gold.
Non-magical full plate sells for 7800 gold.
Magical Armor sells for the values in DMG/UA, with the addition of:
Field Plate +6 - 160,000
Full Plate +5 - 170,000
Full Plate +6 - 220,000
Books that grant a level to a certain class, such as Manual of Puissant Skill at Arms, etc, will sell for 50,000 gold.
Gem of Insight is worth 100,000 gold, possibly more to the right person, but that's a good minimum.
Girdles of Giant Strength:
Hill - 25,000
Stone - 30,000
Frost - 35,000
Fire - 40,000
Cloud - 45,000
Storm - 50,000
Iron Flask - 30,000 gold
Periapt of Proof Vs. Poison - in addition to the static abilities, a periapt can give a plus of 1, 2, 3, or 4 against poisons not included in the special list. The value of 12,500 gold is for the +1 version; the additional plusses add 2500 gold per plus to the value of the item. So, the +2 periapt is worth 15,000, and so on.
Protection Devices (ring, torc, etc.) are worth 10,000 gold per point of protection.
Wands - note that the value of a wand is based on the assumption it is rechargeable, and fully charged. If not fully charged, assume the wand is worth 25% of the full value, plus (charges x 75%) of the wand. A wand that is not rechargeable is worth 75% the book value, times the percentage of which it is charged. The same applies to rods, staves, and other items with charges, in general.
Any item with a wish will sell for about 50,000 per wish.
Any item with a limited wish will sell for about 17,000 per limited wish.