Op is ridiculous. Debit card and credit cards can be tracked easily and not by the account holder, its not paranoia.
Ive paid £5000 for a car in cash before now and ive accepted more in cash also when selling a car. I save in cash and in the bank as who knows when the bank may not be able to give you access to your money, for a myriad of reasons.
If you like using just cards then carry on but there is nothing wrong with cash, many people use solely cash and still pay the right tax. my mum a landlady for one as she is older and she finds it easier to deal with. she uses cash, a triplicate receipt book and a notebook style ledger. and a multi folder for incomings and outgoings needed for her self assessment tax return.
Who are you to say that this is not ok?
In regard to ebay and gumtree, if you bought the item for your own use then its yours to sell in whatever way and for whatever amount you see fit without paying tax, as you have already paid tax aka as vat when you bought the item. The rules are on hmrc website. Different rules apply to business sellers.
Also just because a tradesman offers you discount for cash does not mean he is not paying the correct tax, as offering other payment methods, usually comes with an added cost so cash is cheaper.
All methods should be okay to use and this demonising of cash and cash transactions is just a ploy for greater control and insight into what people spend their money on.