The best I consider to have ever got out of ebay is one day on a whim I decided to see if could slot in a better cpu to my old laptop, it turns out there was 3 models on my laptop made, with 2 difference cpu's, so one other cpu was compatible.
I looked on ebay and someone was selling a few from broken disassembled laptops for £10 each. Brought one and it worked fine (still have the laptop today in another role).
I have checked a few random times since to see if these processors are regurly available and have never seen one on ebay since, I think I got very lucky with the timing.
Dont think I would ever spend £100 on something marked as broken/spare parts usage tho. In my above example the seller did say the cpus are working.