Sale was for 6 dining room chairs, buyer only wants 4

If the listing was for 6 chairs, then that's what the buyer needs to pay for, ie the complete auction. If they don't agree to paying for the 6 they won, then apply to Trademe for a success fee refund and when that comes through, place appropriate feedback and relist so someone that wants 6 can buy them. Presuming of course the listing price was for 6 rather than for 1 and said buyer must buy all 6, which would be against TM rules unless it was a buyer selectable qty multi quantity listing. If buyers want to negotiate for something that's different to the listing it must be done before bidding.

LOL yea, I sometimes get this as well.
Buyer still pays for the entire listing and I often let them take what they want.

Sometimes whats left over can be relisted and sold as well