The whole point of VAT is that there is no such definition of "used product" or "end user". If you buy a book from a store, and have a receipt showing that you paid 1$ + 10 cents VAT, then you later sell the book to someone else for 2$, you owe the government 20 cents VAT on your sale - 10 cents which was already paid, so a total of 10 cents.

The exact law may be more complicated (you may have to pay the full 20 cents now and file for a 10 cents tax rebate; you may not be eligible for this if you are an individual and not a registered organization, etc.) but that shouldn't affect the basic principle.

For example, if you buy a computer for 100$+10$ VAT, and you then use that computer in you back office to keep your accounting, and you sell 1000$+100$ VAT worth of haircuts in the month where you bought the computer, you only owe the government 90$ of VAT even though you are an end user of the computer by any reasonable definition.

If you later sell the computer on to a refurbisher for 50$+5$VAT, and the refurbisher sells it on for 75$+7.5$ VAT, they will only owe the government 2.5$ VAT.

Oh, I think this explains why I was being lied to: "There is VAT on second-hand goods if the seller is VAT registered. Generally, businesses are required to register for VAT with HMRC once their taxable turnover reaches a certain threshold, which for the tax year 2021/2022 is £85,000"

So normal people don't have to pay VAT when selling used things, but larger entities do.