If you’re selling to businesses then you would normally quote your prices EXCLUDING VAT
If you’re selling to individuals you would normally quote your prices INCLUDING VAT
If in doubt then do specify and, if you’re not VAT registered don’t mention VAT at all (I’ve just heard of a startup that was charging VAT even though not VAT registered!)
You may have seen adverts ‘we’ll pay your VAT’ but this isn’t strictly correct. What they mean is that they’ll give you a discount equivalent to the VAT amount so, if you’re VAT registered, you can still reclaim the VAT on the discounted price. (Although most of these offers are on domestic purchases but you might need a new sofa for the office?)