Paid Search, also known as Search Engine Marketing (SEM), has been the largest channel in online advertising by revenue. Yet its causal effectiveness has been difficult to measure. At eBay marketplace we have developed a hybrid Geo+User experiment approach, and conducted a long-running field test over 17-month period to understand the incrementality of paid search on one of the largest e-commerce platforms in the U.S. The experiment results indicate that paid search campaigns drive statistically significant sales lift to the eBay site, and a portion of the direct sales made via paid search is incremental. More importantly, such incrementality demonstrates strong seasonality during the year, and do not necessarily correlate with the paid search spend levels. Our results also show that the new user acquisition lift is higher than the immediate sales lift on the eBay site from paid search campaigns. These findings provide critical insights for paid search and overall online advertising investment strategies.