Is 8gb enough? That will depend very much on your usage. I had the 32gb on my last two phones and it was always full.
If you don't have your music and videos on your phone, maybe you could get by. Personally I've gone to 64gb on my 6+ and am loving the extra room!
Some apps can be in excess of 1gb though, so you will need to be selective.
And yes EBay can be full of scammers, but generally they side with the buyer. Meet in an Apple Store if possible so they can varify the phone as it locked or stolen. And don't pay cash.
Actually I Want Simple Apps Like WhatsApp, Facebook, Some Bunch Of Photos And Small Games Like Temple Run 2 Or Subway Surfers and Thats All. But i Have 16GB iPad Mini To Run Music Or Play High Graphics Games. So What Do You Suggest iPhone 4s 8GB Or Something Else????