1. Since you can't see or touch the item, you might not be sure of its quality, whether it will work, proper size, fitment, and the like.2. Return policies are difficult. Since the cost of return postage is more than the price of the article, you may not be able to return an item. In some cases, the seller just offered to refund my money when I wasn't satisfied. In other cases, you may be out of luck and a few dollars.3. You do have to wait. Since these are sent by parcel post from China, there is no tracking number you can use to follow the shipment. Shipping can take weeks in some instances, mere days in others. Usually you have to wait a week or more, which can be a bit unsettling.
4. Sometimes the product is junk. USB cables that are supposed to be C-rated end up not working for some applications, for example. There are some unscrupulous vendors out there.
5. The language barrier - communicating with these sellers can be difficult as their command of English is limited and often the listings on eBay are brief or confusing.
So for the most part, this has been a real money-saver.