| I've bought about a dozen bikes from auction, normally ex police bikes (Pans and BM's for work).
Sometimes some good stuff and sometimes some complete scrap, definately buyer beware. Sometimes some real bargains, but on many occassions i've seen people get carried away with bidding, paying same price you'd pay from a dealer, then get hit for buyers premium and VAT. Funny when they realise what they've done.
If its got no history or you can't check with the owner, it's gotta be cheap.
In the case of ex police bikes some bikes also have VAT on the full sale price, not just the buyer premium, check before handhand | 
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