We do not have a policy although I've dreamed of drafting one and giving it to HR. In the past, certain individuals were allowed to purchase their laptop when they transitioned off staff. This sometimes happened without IT's knowledge as the transitioning employee made their case and obtained approval from their overseer, and wrote a check for what they determined it was worth. (Not Good).
I really discourage allowing this for primarily a stewardship issue. If we allow an exiting employee to purchase a 2 year old Mac that we could have redeployed to a new/existing staff member, then we have to purchase new equipment, reinvest in something we already owned. To me that just doesn't see like good stewardship.
If the equipment is older and wouldn't be good to redeploy to an individual, then we wipe/reimage and add it to our loaner pool. Staff can check them out while traveling to conferences, training, etc.