so with some help from a few commentors i've figured out how to calculate the return on this paired trade. i sold $100k of ebay short and purchased the same amount of amazon long. while this was neutral in terms of capital used, it probably required around $100k of capital to create. therefore, i'm now up 11.3% on the amzn side and 9.3% on ebay short for a total gain of 20.4%. given that there was no timing catalyst for this trade, i am now tempted to just take the profit. you have to pay short term capital gains on short positions anyway.