eBay is fine for softs but for bats i would prefer to stay away. For most brands there is some problem or the other ...
1. GM => Any cheap deal you see for a new bat is probably a GM - Made in India under license. GM - Made in England is my preferred option.
2. Gray Nicolls => Cheaper option of GN bats are available from Indian sellers and they sell the range which is produced for Indian market. This range doesn't match with the current GN UK or Australia range. Also the bat profile and shapes are quite different.
3. CA/HS/MB/Ihsan => There are so many fakes for these bats and sometime its difficult for someone to even detect if something is a fake. Watch out for sellers from Pakistan and Dubai/UAE selling CA cricket bats. Avoid them as much as possible ...
4. Kookaburra => Watch for Kook bats sold by Indian sellers. 90% chance it would be a kashmir willow with fake Kookaburra stickers.
If you see a H4L, B3, Blank bat etc on eBay, i would say they are safer options to buy. These bats don't come up that often on eBay and even when it comes up i find them to be mostly genuine.