I've only ever sold on Vinted and I've done pretty well.. but some of the offers do take the p*ss!
That being said, I just decline and make a counter offer. I've never been rude about it.
I have the bundle discount offer on my items and recently had a buyer select a few items; that came to £17.50 with the bundle offer, (so they saved about £2-3 pounds).
They then messaged me to ask if I'd take £12.50..
I politely declined, and was surprised when they still went ahead with the sale!
I've gotten so used to people making bundle offers, but then not going through with the sale!