Don't do it. I have lost quite some money on mining contracts (through ebay).
I'm really sorry to hear that, people are so evil now a days. I am a reputable seller on eBay though as I clearly prove on my campaign. I always provide what people pay for.
I bought from 1 new member (so was my fault) and from 2 reputable members.
The first one closed his account and wasn't contactable at all.
The second one (reputable Ebayer registered for 7 years) honored his contract for 2 months out of 12 months. After 60 days you can't file a complaint with Ebay or leave feedback. I had to contact him several times than he told me that his miners stopped working for a while.
For the third one I'm still waiting for the contract to start which has been paid for 3 months ago and due to start in June (also reputable Ebayer.). So we'll see how this goes.