This is a great question. 

If you are doing construction, then the answer is basically “no”, we buy new equipment.  See paragraph (a) in FAR 52.236-5 Material and Workmanship.

If your purchase is just equipment, you have more leeway.  See FAR part 11 and specifically FAR 11.302(b) for “reconditioned” items/equipment.  The DFARS does not provide any additional guidance or different policy.

Also, depending on what you’re buying, see FAR subparts 23.1, 23.2, 23.4 and 23.7.  The DFARS does not provide much additional implementation and supplementation language on these subparts, but again depending on what you’re buying you should review.

Finally, many Service Components have regulations or instructions on the purchase of equipment, be sure to consult yours.