1/ Before bidding email the seller to ask if they are legitimate (I have found that most will 'fess up' if they are not as they don't want negative feedback from an unhappy buyer), if they state that they are legitimate, ask if you can also get proof of purchase from a reliable wholesaler/distributor with your order. REMEMBER - If you are caught with fakes (even if you thought they were genuine) they will be confiscated and you will almost certainly not be reimbursed.
2/ Ensure that if they are being posted direct (this seems to be happening a lot with wholesale joblots of trainers) from the maufacturer overseas that you are not breaking any laws by avoiding paying tax and customs for it being brought into the country. If you are buying to resell from the USA for example you are looking at a minimum 12% tax, 17.5% VAT and possible admininstration fees. If you are not being charged this, then ask yourself why.
3/ There is no such thing as a free lunch! The old saying goes - are you all set to buy 10 pairs of designer trainers for about £200? When they retail individually for between £60-80+. That means you are getting them for £20 a pair and the person you are buying from is STILL making a profit, after ebay and paypal fees! So ask yourself how much he/she got them for in the first place! Is it worth losing your feedback and reputation as a seller (as well as breaking the law) for a few pound? Think about who might have made the fakes, they probably won't have been on a very fair rate of pay if they are fakes!
4/ Check feedback! Don't just look for whether it is positive or negative either, read what it is they have sold! If they have great feedback selling items that are worth buttons and are all of a sudden selling products worth hundreds of pounds, ask yourself why.
5/Where are they shipped from? Do your homework! If they are being shipped direct from a warehouse in a country where the items are not even produced then you are almost certainly looking at fakes!
Let's stamp out fakes on ebay! Fake items are driving down the average selling price, leaving honest seller unable to compete.