Ask the seller where the boots were purchased from, if it was an online retailer, you can check if that website is an authorised retailer on the official UGG website, (i would put it on here but it wont let me) at the bottom you will see 'counterfeit education' and on the left you will see 'retail stores'
If the seller says they got them from a shop, you can also check if that is an authorised shop on the UGG website (you will need the town for this though!) If the seller is selling ONE PAIR of brand new UGGs, they may still have the receipt which you can also check, most of the time they will probably have the picture uploaded already.
There are A LOT of very good copies of UGG boots on ebay, I own a REAL pair purchased from Office in Lakeside, and some of the fakes look extremely similar in the pictures, yet they have more than 10 pairs, and are selling them very cheap?
Please be careful, as REAL UGGS are amazing quality, and really worth the money, don't buy cheap fakes! Even if they 'look real' <-- Look real? Exactly, doesnt mean they ARE real! :) Thanks for reading!