Should I respond to lowball offers at all?
Usually yes, with one line. Plenty of buyers open low out of habit and settle near your price once they see you are reasonable. Ignore only the ones that are obviously scripted, rude or asking you to pay or post outside the platform.
How do I counter a lowball offer?
Pick a counter above your real floor, give it with one reason and stop. "I could do £70 as it's only a year old" leaves room to settle at £65. Halving the gap every message trains the buyer to keep pushing.
How do I deal with timewasters on Marketplace?
Ask for a collection time before you agree a price. Do not reserve an item without a firm time. "Happy to do £40 if you can collect tomorrow evening, what time suits?" filters out anyone who was never going to turn up. If someone asks "is this still available" and then goes silent after your reply, let it go rather than chasing.
Is it ok to say no to an offer on Vinted?
Yes. Decline the offer or reply with a counter in the chat. Sellers are not obliged to accept anything. A polite counter keeps the conversation going if they are serious. If a buyer sends the same lowball twice after your counter, a simple "Sorry, that's too low for me" is enough. You do not owe them a third round.