PESHAWAR, Pakistan – In this town along the road to the Afghan border, you can buy U.S. Army gear, computers and manuals instructing soldiers how to avoid roadside bombs. Traders are coy about where their stock comes from, but much is stolen from trucks carrying military supplies into Afghanistan. Not only does the trade include materials of potential value to insurgents, it also illustrates the challenges of securing supply lines into landlocked Afghanistan, a task underscored in recent days by the closure of the main route through Pakistan and subsequent fiery attacks on convoys.