By Louis Ramirez, dealnews Senior Feature Writer It's no secret that trade-in sites can help alleviate the cost of a new gadget. But as is the case with traditional purchases, if you don't shop around for the best offer, you could wind up getting burned. Shoppers should keep that in mind while considering Best Buy's current iPhone promotion, which offers customers a "free" iPhone 5 16GB when they trade in their iPhone 4 or 4S for a $150 credit. Since the iPhone 5 is currently marked down to $149.99, that credit can just be put towards the cost of the new phone, hence making it free. (Naturally, customers have to be eligible for an upgrade from their carrier and a 2-year contract renewal applies.) The promotion has gotten a lot of attention lately, as it's proved to be hugely popular with consumers. But after a closer look we found that this Best Buy iPhone offer is not the best option for potential sellers. Not by a long shot. In fact, Best Buy's current program is slighting customers by up to $50. Competing Sites Offer $200 or More for the iPhone 4S Best Buy's trade-in service isn't new to the scene, but it recently celebrated its biggest day in the history of its 4-year recycling program when it offered the aforementioned iPhone promotion for one day only. It was so popular, in fact, that the store brought the promotion back last week, valid through June 29. Surely the promise of a free iPhone 5 is helping to fuel this trade-in offer. But after a quick glance at competing trade-in sites, we found that sellers can earn on average about $50 more by selling iPhone 4S models elsewhere. According to Flipsy, a website which tracks the estimated resale price of gadgets, customers who sell a 16GB iPhone 4S via Amazon's Trade-In site can earn up to $242, paid via Amazon credit. However, that rate pertains to the White model through Verizon; most variations will instead receive around $200, or $50 more than Best Buy. Even the Apple Store trumps Best Buy's offer by $31. Likewise, Gazelle is currently offering $180 for a similar 16GB iPhone 4S. To be fair, Best Buy does indicate that it will offer more money if an iPhone is in exceptional condition, and Amazon's top price structure is dependent upon "like new" status; what you get in exchange for your device ultimately depends on its condition. For that reason, consumers should do some diligent price comparing for their specific device. But all that said, Best Buy still has the lowest starting offer amongst all the major trade-in sites. Ultimately, choosing Best Buy will only expedite your purchase of a new iPhone 5, but is that really worth the $50 you're potentially missing out on? We think not. Instead, consider trading in your iPhone 4 or 4S with Amazon, then go to Best Buy and avail of the price cut on the iPhone 5; then use the $50 difference to buy some new iPhone accessories. (It has a different dock after all!) Related dealnews Features: New 16GB iPod touch Prices Will Drop to $190 by September The Price of the iPhone 5 on T-Mobile's Un-leashed Plan Just Went Up $50 How Far Would You Go for a Deal? An avid gadget lover, Louis Ramirez has covered technology for Gizmodo, CNET, Laptop, and various other publications. Follow him on Twitter at @LouisRamirez. Follow @dealnews for the latest roundups, price trend info, and stories. You can also sign up for an email alert for all dealnews features.