The Purchase History on ebay goes back three years to 2015 only. You might be able to find it in paypal under Account Activity and a filtered search, proving you used paypal to fund the purchase.
... I bought something ... in 2011 and I now need a proof of purchase. Where can I find the information?
You might be able to make a special request to ebay to de-archive the information -- but you would need to be pretty specific and have a good reason. eBay only has current year and previous 2 years easily available.