What Apple provides as a trade-in value has no bearing on what the device is actually worth on eBay, Swappa, Craigslist, Facebook, etc.
I wouldn't necessarily say NO bearing since Apple is a player/factor in the resale market. Apple increasing or decreasing its trade-in values can influence how many people put their devices up for sale on eBay, Swappa, Craigslist, Facebook, etc. (versus trading to Apple) which therefore can influence the supply on those sites which therefore can influence prices/values.
Also, some may look to Apple as the wholesale "blue book value" for devices and use those figures to help set their buying/selling expectations. Apple increasing or decreasing trade values could lead to people increasing or decreasing the expected device value in the market.
Apple therefore can potentially have a bearing on the overall market.