What has changed is that you now need to pay VAT on the fees reverb charge you when you sell something.
I've unloaded a ton of my old gear on Reverb and noticed the fees got a bit higher when this new rule came in. I have never needed to supply a VAT number to reverb as I am also a private seller who isn't registered for VAT.
I fear there's been a bit of a misunderstanding Brian, and you have been given misleading information from the support person where the canned answers they gave you didn't apply to your specific situation and were parrotted directly from their screen without being filtered through their grey matter.
All you need to say is "I'm a private seller and I'm not VAT registered so I will have to pay the VAT on the reverb processing and selling fees"
Here's an example of something I sold recently — A nice set of Gotoh banjo tuners that I had on my tenor guitar before I sold it. The tuners sold for £60, plus £5 postage
The Processing fee plus the selling fee comes to £5.81 (£2.56 + £3.25)
The VAT is 20% of £5.81, which is £1.16
The payout is £65 - £2.56 - £3.25 - £1.16 (Selling price plus shipping charge minus processing fee, minus selling fee minus VAT on fees) which is how they arrived at the £58.03 which was deposited into my bank account.