I agree, neither manufacturers nor sellers are doing a good job of highlighting some very open devices.
I have just had the experience of purchasing two cheap android tablets ($100 & $200), which are indeed likely open by most of the criteria which you highlight as being relevant. But this was very hard for me to figure out. The manufacturers for most of these devices do not even have a US presence, so it is hard to get any info form them (Chinese web sites in Chinese). The sellers don't seem to understand this value either and do not highlight it. The only way to figure it out is to correctly identify the device (which can be very hard in itself with all the lookalike models), and to find the right forum for the device, likely on slatedroid or the XDA forums. On these forums, you will need to spend hours reading many posts about the device to eventually conclude that perhaps, the device is indeed open (surprisingly most of them are).