Aren't second hand goods exempt from VAT as it's already been paid once?
Thats very nearly true. According to HMR&C, you dont actually pay VAT on second hand goods, only based on a margin of any 'value added'; eg, any mark-up. If for example AG sold a second hand putter on at the price they paid for it (eg, £15), no VAT would be recoverable; if they marked the price up to £60, VAT would only be recoverable on the difference, £45.
