You have ebay buyers protection if you dont get what you want so there is little risk to buying i see. The bigger risk comes from selling on ebay. My tips for buying on ebay though if you do/can:
1.)Use ebay case system if you do not get what you paid for
2.)Use paypal cases as a back up if needed.
3.)Pay via card and not your paypal balance so you can do a bank chargeback if needed.
4.)Buy in small amounts, there is no reason to buy a large amount in 1 go, i dont believe it could be cheaper this way else the seller is maybe a scammer - not that they can really scam you anyway if you follow rules 1 -3, it just means your time wasted is at risk.
But yeah all in all i might actually buy small amounts of bitcoin on ebay if possible tbh. Dont see why not. Depends on price more than anything.
The problem with selling on Ebay is that it requires Paypal to purchase and recieve the money, however it has been known that Paypal is very easy to open a dispute and get your money back through the dispute by doing a chargeback. Up to 30/60 days later. Which is why trading in Bitcoin with paypal is not recommended.
