There are certainly worse things you could put on a resume. It sounds silly at first, but I'd feel better about hiring someone who had hundreds or thousands of satisfied customers with few or no unhappy ones.
It may be for protection of other customers. A bad actor could order some of those devices, flash the firmware with an evil version(or somehow modify it), and then send it back to dell.
(Shrug) So send it back to the factory for reconditioning. Charge the customer a reasonable restocking fee to cover the associated costs, if it happens enough to be a problem.
What? The factory can't restore the firmware to as-new condition? That's a bit of a problem in itself, isn't it?
It was just a guess - I have no real idea why Dell wouldn't allow returns on those items. And firmware was just an example. You could also solder in a completely separate computer if you really wanted to. Then you need to open the whole thing up and do visual inspections, reflash the firmware, test it, etc. It may not be worth it for $500, or cost close enough to $500 that they just don't allow returns.