click here is worth a read. Also check out the vendors feedback rating the more positive feedback the better But as to KNOWING if what they say they're selling is what they are actually selling. Not until you open the box. Also worth reading click here
Always check who the seller is. Look at their "Feedback Score" (see what past customers have said about them. Never pay in cash. Get yourself a PayPal account. Not everyone on Ebay is a fraud, in the main they are to be trusted, having said that I have been done once but I should have checked their feedback before I bid, Won't get done again that way. Good luck.
For every dishonest ebay trader there are loads of honourable ones. Of course they are in it to make a profit but that doesn't mean all are crooks. Common sense has to prevail as far as prices are concerned - if anyone is daft enough to pay £4.50 for a tesco carrier bag they deserve all they get!
A lot depends on what you are buying - supposedly genuine jewellery, watches etc., could quite easily be fake, so you'd need to be a bit more careful.
Sellers are awarded rating stars - check these out for feedback from satisfied or otherwise customers.
If you are worried about who you are dealing with it is easy to check the sellers feedback. Also if you require clarification of any detail in the item description you can email the seller for further details. Save any emails you send or receive in reply and DO NOT BID if you are unsure of the seller / item for sale.