VTR - can someone explain

Several points in your post, but the main one is have you actually read the VTR policy ?

You do not need to send all by a tracked method. Untracked is fine for lower value items under £20, but you would be wise to send anything over £20/£25 as ‘signed for’ for the increased compensation cover.

Why ? They are the same labels as are bought at the PO, and for small parcel size packages they are cheaper on Amazon (£3.00 v £3.20 at PO), so depending on how many you send the 20p soon add up.
Also, Amazon will automatically validate all your shipments, confirming shipment and you will have a VTR of either 100% or N/A - both of which are good.

It does not mean actually use physical stamps, it simply means that when you are confirming shipment that you choose the ‘Stamps/Franking’ option from the second dropdown, and not just 1st/2nd Class, as they are no longer exempt.

Are these all your own personal second-hand books that you are selling, or do you buy stock to sell on ?
When you say private seller, do you mean you have the Individual Seller Account type (the legal entity), or do you mean you are registered with both HMRC and Amazon as a Business - sole trader ?

If you are ‘dealing’ then you should be registered as a business.