After reading other peoples guides on fake merchandise I thought that id warn you that I too was conned with G-Star fake jeans costing 30GBP from a seller on eBay, I was fairly new on eBay and had a suspicion that they might not be original, but after reading feedback that was given from his other buyers they all seemed pleased with their product and stated that it was "genuine" and that they were "very happy with the fast and speedy service". He clearly stated on his page that he does not sell fake merchandise what so ever... The jeans were sent from Thailand so I guess you should be wary when you see that as the product location. Also don't be fooled by positive feedback of others or the sellers rating as I'm sure that can somehow be fixed to be in their favor. When the jeans arrived (about 2 months later I may add!) they were stuffed into a tiny box, folded up into about 10 folds! Once I opened them up and checked them out I could instantly tell that they were fake. I looked at the label on the inside which had some Chinese or Thai writing on it, the quality was also very flimsy. The over all quality of the actual material was in two words; very THIN and CHEAP (much like a thicker than average dish cloth).
So in conclusion, the chances of you finding original G-Star Jeans on eBay that are ranging in price from 25 - 40 GBP are very slim, in most cases they most definitely will be fakes. Don't be afraid to ask the seller questions about the item, however clear the item seems to be described on their page. This in turn could help you out in the long run as proof in the event that he claims they are definitely original when they clearly are fakes. Believe me this will make your life a lot easier if needing to return the product or if needing a refund.
I appreciate that there are alot of us out there that would prefer not to fork out hundreds of pounds at a time for a designer product and would rather search on eBay for a generous seller. Just make sure you can tell the difference between a good samiritan and a wolf in sheeps clothing (some may say there is no difference lol).