* Q1 EPS $0.13 vs est $0.03

* Q1 sales fell 17 pct

* Q1 comp sales fell 17 pct

* William Blair upgrades stock

* Shares up as much as 18 pct

(Adds conference call details, analyst comments)

June 19 (Reuters) - CarMax Inc (KMX.N: Quote, Profile, Research), the largest U.S. retailer of used cars, posted a better-than-expected quarterly profit on stronger sales execution, prompting at least one analyst upgrade, sending its shares up as much as 18 percent.

Comparable store used-unit sales fell 17 percent during the quarter, sequentially better than a 26 percent fall in the fourth quarter of 2009.

"The majority of this improvement was due to better sales execution although traffic did increase modestly during the quarter," Chief Executive Tom Folliard said on a conference call.