Customers browse in a used car lot on Feb. 15, 2023 in Glendale, California. Mario Tama | Getty Images News | Getty Images DETROIT — Used car prices fell last month for the first time since October as gas prices rose amid the war in Iran. Cox Automotive's Manheim Used Vehicle Value Index — which tracks prices of used vehicles sold at its U.S. wholesale auctions — decreased 1.6% last month compared with March and were up 1.8% compared with the same month a year earlier. Cox said affordability remains a key concern for buyers, which is driving increased demand for older vehicles and all-electric vehicles at Manheim auctions. Gas prices at the end of April were up $1.12 per gallon compared with a year earlier to a national average of $4.30 a gallon, according to AAA . They've continued to rise since, with the national average hitting $4.56 as of Thursday.…