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.
money.stackexchange.com/users/44366/Harper - Reinstate Monica
One method you can use to find the value of some items is to look for similar items on eBay.
When doing this, there are a few important things to remember: The comparison item should be in the same condition as yours.
Don’t use the price of a new item if you did not donate a new item.
If you are looking at an auction that has not closed and has no bids, you aren’t seeing a true value of the item, but only the asking price from the seller, which might be too high (the item might not sell).
You want to look at closed auctions that resulted in a sale.
If the final sales price looks abnormally high, don’t use it.
There might be some sort of problem/fraud going on with that auction that made the price higher than it should have been.
money.stackexchange.com/users/10997/Ben Miller - Remember Monica
The rule of thumb is that you can deduct the property’s fair market value at the time of the donation.
Fair value is what a willing buyer would pay for and what a willing seller would accept for the property.
Some charities provide a guide for this.
For example, on their web site, the Salvation Army offers a "Donation Value Guide" which provides a range of value for commonly donated items (high and low estimates).
Donations worth $5,000 or more require a formal appraisal from a qualified appraiser.
This does not apply to marketable securities since their value is known.
money.stackexchange.com/users/67285/Bob Baerker
If the place you donated the item to has an online presence (and yes goodwill does) you might be able to see what they are selling a similar item for.
If you use this method then make a screen capture and save it with your other tax records.
The idea is to come up with a reasonable value using a reasonable method.
money.stackexchange.com/users/5414/mhoran_psprep
You are supposed to find the "fair market value".
Ways to find a fair market value for used merchandise include: Get a list from a charity.
Many charities publish lists of estimates of fair market value for a variety of items.
It doesn't have to be the charity you actually donated to.
If you gave to Goodwill and they don't list a value for this item, see if you can get a value from the Salvation Army or Habitat for Humanity or whomever.
As others have noted, find a similar item on eBay or in classified ads.
Failing that, see what percent of their original value used items have in general.
If items of similar durability and similar age tend to be worth, say, 30% of the original price, then use that percentage.
Unless the items are extremely valuable, the IRS isn't going to quibble if your numbers are reasonable.
If you claim that a back brace that you bought new for $200 five years ago is now worth $10,000, yeah, I think the IRS may question that.
But if you claim it's worth $50 and you have some plausible basis for that number, the IRS isn't going to say it's only worth $45 and charge you with fraud.
Frankly, I'd say when in doubt, guess low.
I'd rather miss out on $20 worth of a deduction than risk trouble in an audit.
I suppose that depends on how much these items are worth and how rich you are.
money.stackexchange.com/users/14118/Jay
I made a donation to Goodwill that included 2 back braces and a foam wedge used for exercising. How can I estimate a value?   Most of the valuation guides cover clothes / appliances / furniture / housewares just fine, but I have no idea how to find a valuation for medical-related items that I no longer need.
charitable donation values for medical devices/gear
money.stackexchange.com
<united-states><tax-deduction>
money.stackexchange.com/questions/104637
money.stackexchange.com/users/1631/Jason S
Nobody will check this valuation unless you are audited, so you just need to be able to justify the amount. You can look for similar items for sale on eBay. The IRS isn't going to quibble if your numbers are reasonable. The rule of thumb is that you can deduct  the property’s fair market value at the time of the donation. To find the value of some items, you can look for similar items on eBay. But donations worth $5,000 or more will require a formal appraisal from a qualified appraiser. Unless the items are extremely valuable, the IRS will not be interested as long as your numbers are reasonable.
Nobody will police this until audit time, at which point you will need to justify your valuation to the IRS.   The rule of thumb is that you can deduct  the property’s fair market value at the time of the donation. To find the value of some items, you can look for similar items on eBay. But donations worth $5,000 or more will require a formal appraisal from a qualified appraiser. Unless the items are extremely valuable, the IRS will not be interested as long as your numbers are reasonable.