This (currently) requires a hex editor. The common\item\itemData.itm file format is as follows (using the current version as an example, with Potion for the item data (entry #2 in the file):
Header (6 bytes):
AE00 Unknown - version?
E8030000 Number of entries
Item Data (32 bytes x Number of entries)
01000000 ID
00 Sub type? (See below)
00000000 Type/Container (See below)
00 Rarity-1
0A Carry Limit
0A Unknown (Usually = Carry Limit, but not always)
0200 Sort Order
08000000 Unknown (Appears to be a bitmask - at a minimum, handles the starburst in the corner of icons)
05000000 Icon ID (Icons are found in ui\common\tex\cmn_item00_ID.tex, in a 16x16 grid)
0200 Icon Colour ID (Unsure where the colour codes are stored)
08000000 Sell Price
42000000 Buy Price
Type IDs
0 - Item
1 - Material
2 - Account Item
3 - Ammo/Coating
4 - Decoration
Sub-Type IDs (when ID = 3)
1 - Ammo
4 - CoatingItem names and descriptions can be found in couple of places, the best being "common\text\steam\item_eng.gmd" (has more info than "common\text\item_eng.gmd"). The entries in those files are ordered the same as the entries in itemData.itm. Disabled items will have a name of "Invalid Message", "Unknown", or have an Icon ID of 0 or 255. This does not mean the remaining items are available (yet) in the PC version however. The upcoming DLC items have proper names, and icons, but obviously don't work (and it's not issue in the unmodified game, since they don't drop anywhere). Also, not every item has correct buy prices. The shop mods which enable everything, can also give infinite Zenny because of this. Demon Ammo (as an example) can be sold for 9z. When added to the shop (without fixing the price in this file), you can purchase it for 1z.