I think the most useful thing would be mod-related to the phones in question. Many phones have specific mods to improve them, and sometimes they require some extra equipment - foam, tape, tools, others. Or maybe some pads (if you are doing this large scale, original pads could maybe come out at the price mark). Many recommend to get the Shure SRH840 pads for SRH440 cans, for example.
To generalize, I'd want something to enhance the phones, rather than some gimmicky stuff on the side. The buyer can choose to mod, or keep them original and untouched.