Nobody polices this until audit time, at which point you will need to justify your valuation to the IRS. A single error among many good valuations is unlikely to result in action, but a pattern of donating items and overstating their value will.
Go to eBay. Search for items like yours.
On the left edge, will be a number of filters. You want "Completed auctions".
You will get a mix of prices in red (did not sell) and prices in green (did sell). The red ones are useless. Check out green ones until you find ones comparable to the condition of yours. Hit "Print... make PDF..." And save the PDF in your tax records. Do this for 2-3 comparables.
Do this when you file your taxes, or better, when you donate the unit, since prices on rapidly-losing-value items like iPhone 6's will change much between donation and audit time.
This will show the due diligence that the IRS is looking for.