Whether or not you have to pay tax on any profit isn’t about how much you make or how many items you sell, but whether you would be deemed to be ‘trading’. Obviously the more you sell and the more profit you make, the more it looks to HMRC that you are trading, but there’s no real straight answer to it. If you’re genuinely selling some left over items that you don’t want, then that’s not trading. If you’re buying edits/calendars with the aim of selling a large proportion of it and/or with the aim of making profit, then you are probably trading.
Re the low vinted offers, I don’t even respond to them anymore, not even to reject. They’re the type of people who will order, receive the parcel and then start with ‘so how do I know this is genuine?’