It's not as always as simple as this, folks.
Most companies, like mine, have an "ordinary" VAT registration. (I don't know whether there's a more technical name for it.) They can reclaim VAT when buying goods, but if they subsequently sell those goods then they must charge VAT on the selling price.
However, many companies who do a lot of buying and selling used equipment will operate under the "margin scheme" for VAT. This means they can't claim back VAT on goods they purchase, and they only charge VAT on the proportion of the selling price which represents their profit.
I know for a fact that
MPB Photographic, who were mentioned earlier, operate under the margin scheme.
Suppose you buy a used lens from me and another from
MPB, and you pay £660 each time. I have to charge VAT on the full selling price, so the £660 you pay me is actually £550 + £110 VAT. If you're VAT registered, you can reclaim the £110. However MPB only charge VAT on their profit, so how much that is depends on what they paid for the lens. If they bought it for £600 then the selling price includes £60 profit which is £50 + £10 VAT, so the total selling price is £650 + £10 VAT. If you're VAT registered you can reclaim the £10.
In either case you should get a VAT receipt showing how much VAT you've paid. Of course, if the retailer uses the margin scheme then you won't necessarily know how much VAT there is before you buy.