A "clever-er" way of bargaining for non-negotiable items on Carousell

Not sure if you have encountered this on Carousell, but here's a little trick some buyers might use to negotiate for lower price in a sneaky manner.

Buyer : "Negotiable?"
You : "Nope, sorry. Prices are already very low, so it's fixed."

If you sell a few items, this buyer will then ask you whether you can give a discount if he buys both item 1 and 2.

Let's say both items cost $10 each.

"Well sure," you say. "in this case, how about $2 off in total?"

"okay, I will transfer payment later" says the buyer.

1 or 2 days later...

"Hi, sorry I decided that I need only item 1, will pass on item 2. I have transferred $9 to your account."

If you are not sharp enough, you might just miss it and let the deal pass.
But well if you are sharp or your margins are really tight, you will realise that this buyer has on his own accord, taken a $1 discount even though he is just buying one item (a previously non-negotiable one at that)!

What do you do? 

Cancel the deal?
Ask him to top up the $1? He might just say, alright, refund me the $9 I transferred you. Do you want the hassle of transferring back or just let it pass since it's just $1?

Clever isn't it? You can try it as a buyer, it might work!