sheet to automatically add vat to the commercials sale price

Hi
Theres a bit more Im afraid
Ive attached a sheet

So Comercial vehicles attract vat on the sale price at 20%
We dont pay VAT on any profit

Cars however are different, we pay vat on the gross margin from the purchase price to the sale price ie
Bought for £9000 sold for £10000 therefore we pay £166.67 as a cost
However if we buy for £9000 and sell for £8500 we dont get vat return for the £500 it goes as zero
(yes we do lose money selling cars!)

So on my sheet Ive been manually adjusting the costs of commercials by entering a minus figure equal to the vat on the sale price (o13)
Then zeroing R13 Ideally I would like this to be done auto

On cars the issue I have is I have to manually zero the vat if it shows as a loss !

Hope this makes sense and someone can help me please

Thanks in advance