Just because the board has power dows not mean it can boot or post. Look at the keyboard, does it light up, does the capslock or numlock light up. If it does than thats a good indication that the board has some level of working function. If it does not then chances are the board can not take any form of input, such as flashing.
Reasons a board wont post could be
its a dead board
dead, incompatible or missing cpu
bad or incompatible ram
bad, or not connected correctly PSU
Short in one of the connectors such as USB or PS2.
Question is why wont they post. I would remove all non esentials and try to post. Just board, chip, heatsink, 1 ram stick and key board, and of course power, unplug all other connectors and fans from the board. No ide no usb no upfront audios. It should post test the ram and look for the boot drive.
If you got the board 2nd hand did some one take out the CPU and put the heatsink back in place so it did not get lost?