First, keep in mind that it can sometimes be difficult finding decent suppliers for computers, electronics and cell phones (as well as cameras and video game systems). If you're just starting out selling online, I would stay away from these products.
Not only is it difficult to find good suppliers with decent prices for these products, but you'll face heavy competition among huge numbers of other people who are selling the same products at such low prices that you'd never be able to compete with them on pricing if you're dropshipping.
There are too many sellers who are already buying these products in huge lots of thousands at a time, so they can then resell the products for a fraction of the price you would pay for dropshipping the item.
Additionally, I strongly discourage you from using any suppliers you find on DHGate or Alibaba, as the vast majority of suppliers listed on these 2 websites are Chinese, and a large percentage of them sell counterfeit items. I just wouldn't risk it, particularly for high-tech items like these.
As a side note, I'm not sure why you're interested specifically in looking for suppliers at trade sites, but I wouldn't recommend it. Trade sites in general are breeding grounds for widespread fraud -- there's too little monitoring and regulation on most trade sites.
Next, even if a supplier on one of these websites says the products are OEM, it doesn't mean they really are. They probably aren't. So to answer your specific question, NO, these are not as good as the original items, and in fact are most likely illegal counterfeits. You would be breaking the law as well if you were to resell these items online.
Which brings me to one way you can tell whether or not a supplier is legitimate...if they're located in the U.S., most likely they're legitimate and sell legitimate products. As far as wondering whether or not they will actually ship the items to the buyers, you can usually be pretty safe if they have good reviews written about them by other people, such as the reviews you find regarding most of the suppliers listed here on Salehoo.
Additionally, if you pay the supplier using PayPal or a PayPal debit or credit card, PayPal will protect you/your buyer if the supplier doesn't send the item, and you'll receive your money back.
You didn't indicate whether or not you looked in the Salehoo directory for any of these products yet, but if not, that would be one of the best places to start.
I took the liberty of searching for these products in the Salehoo directory, and here's what I found:
1. Waterproof TV - I found one supplier listed on Salehoo for this type of item, www.inesun.com, and they ARE Chinese, but the fact that they're listed in the Salehoo directory means at least that Salehoo has investigated them to a certain extent to establish their legitimacy. Still, no one has yet posted a review about them on Salehoo, so I'm not sure if I would personally jump right into doing business with them.
One thing you can try that may help, however, is to order just one product from them to test its quality and legitimacy. When you receive it, take it to a retail store or repair outlet that specializes in that brand or type of product, and have them inspect it for legitimacy.
2. Google Android apad (the correct term is "apad", by the way) - I didn't find any suppliers listed in the Salehoo directory for either "android" or "apad", but if you email Salehoo customer support, they'll be happy to look for a supplier for you that carries this product.
3. Blackberry Torch 9800 cell phone - I didn't bring up any supplier results on Salehoo for "blackberry torch 9800" or even "blackberry torch", but just searching the word "blackberry" yielded several different suppliers, most of whom have had reviews posted, and a couple of them with excellent reviews:
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Good luck to you!