You can't refer to ebay like a single unit , it is just a mean to a deal between a seller and a buyer and provides some assurances for the transaction.
There are different sellers from different countries so you must treat each deal as an individual case especially with so many different shipping services used.
In general you should expect transit periods up to a month especially from china.
If you buy from a seller located in your country then obviously it would just take a few days.
In general I've been using ebay for a few years and the prices you can find are usually low but there will be cases of fakes , items that will never arrive, items not as described or items not properly packaged that will arrive damaged.
You can limit some of these cases by choosing sellers with good feedback but you can't prevent them completelly.
The good news is that ebay and paypal can help resolve such cases and receive a refund or a replacement.