Personally I have bought both, motherboard and cpu on ebay and wasn't dissappinted. To my mind buying those items on ebay is ok especially when they aren't pricey but new ones aren't expensive on newegg too.
Don't buy GPUs, especially models massively used in mining, there is high chance that will be used in mining and worked on high hashrate/overclocking.
P.S See sellers rating and reviews of product, how much he sold, if he accepts returns and etc.