Once upon a time, the name uggs was used to mean a certain type of "after-surf" boot, invented by our surfing brothers (and sisters!) Down under. Similar in style to todays brand-name product, these ugg boots were made of pure sheepskin (and there are a LOT of sheep in Oz, enough for everyone to have a pair really) and were designed to chuck on your bare feet after surfing, an ideal piece of footwear to keep your tootsies warm on the beach after a day in the sun, because it gets dark and chilly quite quickly at the coast in Australia because there are no clouds to keep in the heat. These boots kept your feet warm without the need for socks, due to the lovely woolly lining, which also dries quickly. With a chunky sole, you'd only need one pair to last for years...
these boots inevitably found their way to the UK, and became popular with surfers down in Cornwall and Devon. The Celtic Sheepskin Company started making these boots, the same style as today's famous ones, and trademarked the name Ugg. Ugg boots were officially born. They were made here, in England, although of course invented in Australia.
The trademark Ugg was sold on and is now owned by Decker, under the Ugg Australia brand. These are the Uggs that cost over £130 and are the ones that people refer to as genuine Uggs.
Because of the gernirc name ugg still being used for this type of boot, as well as the fact in Australia there are a few types of boot which actually have the name Ugg on their label, many Ebay sellers do claim their boots as being 'genuine Uggs', genuine australian Uggs' or Genuine made in australia Uggs" (There is actually a type of Ugg from Australia which has a label similar to the Decker Ugg australia boot, with Made in australia Ugg" on it, and this confuses people.

The problem overboots made in china now no longer applies since like many large companies, Decker is taking advantage of cheap labour and unregulated labour laws/conditions in China. Also like other american companies, such as Abercrombie and Fitch, Gap etc. Decker do not actually supply the materials and get the factories to make the boots for them. not so: the factories have to BUY the materials, the sheepskin etc, and make the boots and then SELL the completed boots which are up to standard back to Decker. this of course means that only the best ones get the factories their top price (which in in any case a pittance...a worker may get the equivalent of £10 per WEEK, while you pay over ten time this for one of the hundreds of pairs he has made.)
The reason it is impossible to tell, on ebay, whether a pair od the genuine-looking Decker boots is actually really one of their approved pairs, is because all those boots which the Chinese factories have made, but which do not meet the standards which will allow Decker to buy them, have to be sold somewhere to allow the manufacturers to recoup their costs. They are sold through many avenues, mainly on the black market, and there is no real way of telling whether they are geuine or not, other than by close comparison of their labels with those of the approved ones. Only the approved Fecker uggs will get the "real" labels. The other ones' labels will look remarkably similar , but you can bet they are not up to scratch.
so...what if you don't mind? Well, apart from the fact you will be paying too much, since the 'seconds' are just not worth the money, you are also adding to the practice where people, often very young people, work their backs off for about 14 hours per day, 365 days a year. If Decker want  good quality boots they should actually supply the materials themselves and regulate the factories, put some quality control in place,. or have the boots made in australia where the sheepskin is from, so that your boots haven't got a carbon footprint as big as the moon. it is ridiculous that sheepskin from australia is flown to the States, then sent to China, then made into the boots and flown back to the States, and might end up back in australia for sale after travelling thousands of miles!!
Watch out for fakes. compare the label close-up of one on the boots at uggaustralia.com. you will always get close-ups of the labels if the seller is genuine. If in doubt do not bid.
If you do not mind whether your Uggs have the Ugg australia name, then fine, but remember that these Australian generic uggs, even the ones called Ugg, are sold for about a thrid of the price of the genuine Decker Uggs, and so on ebay you should not expect to pay more than about £45 to £50.
If you want really good ugg-style boots, but not necessarily Ugg, try the Celtic Sheepskin company (U.K) which is where trademarked Uggs were first made. They use good quality sheepskin and their boots are machine washable. They come in about 15 colors and this includes kids' sizes, for around $90. Or try Bearpaw, Emu Ridge, Kiwi sheepskin or Warmbat.
If you suspect someone is selling boots under the Ugg name but they are not Uggs, report the seller. always look at feedback and how many previous items the seller has sold.