Check the serial number prefix, if it is "DM", this is the old model BFR, and is of significantly lower quality than the new model, which was introduced in early 2000. (This might be the reason for the cheap price.) The new prefix is "J" (I think), and the guns are much higher quality.
That being said, my BFR is a "DM", and it is working perfectly after some tweaking. Check the forcing cone for cracks, and check the barrel/cylinder gap, which should be very tight (around .002, if memory serves me correctly). These were the main trouble spots for DM models. The first will require replacement of the barrel, and the second usually requires replacement of the cylinder. Magnum Research may honor a warranty claim, but I am not sure if you are not the original owner.
Other than those factors, it is a GREAT deal on a great gun.