This can be a tough call. I own an arc Alpha LT made in 08. As i recall, production was already off in China by 08, yet mine was still made in canada. I recall hearing that the company still retains canadian manufacturing on some of their high end items. Perhaps you can call the company and ask them if they still make the theta ar in Canada?
And in general, a good way to avoid ebay fakes is to
1) check item location. If its from any part of asia, i immediately walk away.
2) Check user feedback, and other items that they have up for sale. If there are multiple listings for the same item, theres a stronger chance that something is fake
3) look for real photos as opposed to stock manufacturer photos
Perhaps you can link me to the auction? I can take a look and give my honest opinion. I've seen and handled a number of arc jackets, so maybe I can help
Also, since joining the lightweight community, my hindsight has become a lot clearer, and can honestly say that many arc products are overhyped (especially in terms of quality, since production has moved offshores) or over built. My alpha SV only comes out when i mountaineer/snowboard….its just too heavy and not as breathable as my alternatives. For normal rain, I prefer anything made of eVent. If im trying to go ultra light or theres only a very slight chance of rain, I look at jackets such as marmot mica/essence. So depending on your application, it maybe be worthwhile to check out the alternatives