The camera (type, quality, flash, natural/artifical lighting etc) can also affect how a pigment look.
IMO: 'Authentic guarantee' can be - and if i rememeber correctly has been - used on fake auctions too, it can be a ploy to entice/deceive buyers into thinking the seller is more trust worthy.
Its very hard for a buyer to provide concrete evidence that a pigment is fake - alot of people cant collect such evidence easily (photographic evidence of real next to fake, statement from a MAC MUA etc), and even if they could the seller can argue that the pigment that someone says is fake wasnt sold by them - how can you have evidence to back this up? Its word against word. If they had good fakes then the authentic guarantee can be a great selling tool for them.
Location, seller communication, feedback score (total, how much is about MAC - how long have they been selling MAC for, negatives and neutrals, along with comments), auction wording and product photos are a better judge of whether a pigment is more likely authentic. However the photos in auctions may not be the sellers, or may not be of the actual product you receive. For me now its a case of its more likely fake unless proven authentic on arrival - sadly this doesnt seem too paranoid given the state of ebay at the mo.
