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Many of us have had similar frustration with various models over the years. For the T400, assuming the seller can't identify it, the best way to determine the screen type and resolution is to request the exact 7 character model number for the machine. It can usually be found in both the first BIOS screen and on the bottom on the serial number label. If the model happens to be a CTO (custom to order, meaning the last three characters of the model number are CTO), then you will also need the 7 character serial number. From there, you can look up the exact build parts via the Lenovo support pages. Finally, you can cross reference the exact LCD screen part number with the T400 Hardware Maintenance Manual (HMM) - or other online resource to be sure about the manufacture, type and resolution of the LCD.
All that being said, at any point since the machine was sold, the LCD and/or entire LCD assembly may have been replaced or swapped for various reasons (desire, damage, failure, etc.) So, in the end, if the seller can't confirm the specifications, you may not know until it arrives!
The silver lining is that sometimes you get more than you expect! I consider all of these variables part of the sport of eBay!
T510, i7-620m, NVidia, HD+, 8GB, 180GB Intel Pro 1500 SSD, Webcam, BT, FPR Home
T400s, C2D SP9400, Intel 4500MHD, WXGA+, 8GB, 160GB Intel X18-M G2 SSD, Webcam, BT, FPR Travel
Edge 14 Core i5 | Edge 15 Core i3 | Edge 15 Athlon II X2| Edge 15 Phenom II X4