I'll get straight to the point. The only way to guarantee a G-Star item is genuine before you buy it, is to ask the seller to provide a picture of the label from inside the G-Star clothing item. It is as simple as that.
If the seller can provide and the picture shows the original G-Star logo label, make, model number and size etc - it is genuine. (Back this up by Googling the item and checking the item against a shop item - or on the G-Star website / www. g-star .com)
If the seller can't provide you with a pic, you have to ask yourself how they have been able to upload an original picture of the item in the first place. The seller should send you the pic asap.
No pic = no real reason to buy.