always use the ask a seller a question option if you use ebay - even if it's just to ask a dummy question - this at least checks the seller is partly in control of the account. and watch out for adverts which have in them 'contact me at
xxxxx@yahoo.com as I will not answer if you ask a seller a question' also watch out for dodgy spelling like 'dont contact me trough ebay'.
if it looks like a bargain fair enough but if it looks too good to be true then perhaps it isn't true!
one scam was a bloke selling top up cards for mobile phones with £100 on them but at £50 etc - this was too good to be true. it turned out to be a hijacked ebay account.
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and if you have doubts - ask for a second opinion off someone you trust - show the advert to someone.