Aiming to get a greater share of iPhone buyers, Best Buy is planning another promotion, this time guaranteeing at least $100 credit with the trade-in of any working smartphone. That makes the base-model iPhone 5c free with any trade-in, and the 16 gigabyte iPhone 5s available for $99 (though customers may still owe sales tax on the new phone, and are responsible for any activation fees). And, of course, customers have to be eligible for an upgrade to get those iPhone prices. It’s the latest in a series of offers from Best Buy and other retailers looking to convince would-be iPhone buyers to go with their store. The promotion will run Oct. 13-19, a Best Buy representative told AllThingsD. Best Buy Mobile stores, though, will start offering the promotion on the 5c starting on Thursday. If the iPhone model a customer is looking for is not in stock, they can get a rain check.