TL;DR: Go with Glyde or Swappa
Based on our analysis, Glyde and Swappa tend to offer the highest trade-in values. For instance, for a 32GB Galaxy S7 on Verizon, Swappa offers $349 and Glyde $304. Compare that to just $135 for Gazelle and $160 for Amazon.
It was a similar story for the Galaxy S6 Edge on T-Mobile. Swappa offered us $364 for a 32GB version of the handset, compared to just $80 for GameStop and $96 on Wal-Mart.
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In general, we noticed that Sprint Galaxy phones have less trade-in value than other carriers. Plus, you aren't likely to get as good deal from Gamestop or Wal-Mart.
Note that 64GB versions of the S7 and S6 have been more popular overseas and not sold through U.S. carriers, which is why you see so few of them being accepted as trade-ins. The Galaxy S8 will likely offer 64GB in the U.S., though.
Galaxy S7 (32 GB)
The best trade-in value for last year's S7 comes from Swappa, which will give you $416 for an unlocked 32GB S7. Swappa's values are the best across the board, though Glyde comes close, offering between $304 and $340 depending on which carrier your phone is tied to.
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Galaxy S7 (64 GB)
Not many outlets will buy back the 64GB version of the Galaxy S7, but an unlocked version fetches the best return from Swappa at $416. The only other retailer offering trade-ins for the 64GB model is Next Worth, which will pay as much as $250 for the AT&T version of the phone.
Galaxy S7 Edge (64 GB)
As with the S7, you'll find few trade-in options for the 64GB version of the S7 Edge, though Swappa offers the most money.
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Galaxy S6 (64 GB)
While Swappa offers the biggest value for the unlocked 64GB Galaxy S6, Glyde has the best trade-in value for each of the carrier versions of that phone.
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Galaxy S6 Edge (32 GB)
Swappa's the choice here, offering $348 for an unlocked 32GB Galaxy S6 Edge. It also has the best trade-in values for carrier versions, giving you between $266 and $364 back.
Galaxy S6 Edge (64 GB)
Swappa will give you $365 if you trade-in an unlocked 64GB of the Galaxy S6 Edge. It also offers the best trade-ins for carrier-tied versions of the phone, though Glyde is the only other outlet to offer trade-ins higher than $200 on those models.
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