When you donate an item, you can deduct the amount that you could have sold the item for on the open market. For instance if you donate an '83 Chrysler LeBaron, you cannot deduct the $11,800 sticker price, but the $200 "guy on Craigslist will pay that" price. Or arguably, the $900 "State smog buy-back program will pay that" price, but that argument is between you and the IRS.
So if you could seriously get $100 for the items you donated, then you can deduct $100.
Let's say it's the Habitat for Humanity Store and I donate 75 120mm square junction boxes, a bit rusted. I claim $100 donation, that's fair. I cannot claim the $3/box my electrical supply charges for new ones, nor the $7/box Home Depot charges for new ones.
Something interesting happens with appreciated assets. Your charitable deduction is the appreciated cash value at the time you donate it. But you didn't sell it, so you don't pay capital gains tax. The charity sells it, paying capital gains at their tax rate of zero percent. So don't sell things and donate the cash. Donate the things. Your charity probably can't handle that, so donate it to a donor-advised fund (DAF), then 'advise' the DAF to support your charity with the proceeds. The DAF has an office of people that do nothing but liquidate "things".
Separately, the purchase.
You can take a tax deduction for the value you pay for an item over and above its fair market price.
Suppose back at the Habitat for Humanity Store, I buy a $300 chandelier for $100. No tax deduction for me! It is not tax deductible merely by being a retail sale from a nonprofit. If that were the case, lots of nonprofits would own car dealerships!
Instead, suppose I buy 250 plain receptacles for $1 each. The things sell for 60 cents at Home Depot and accordingly, I haggle them down to $150. But I like HfH so I agree to pay $250 anyway. If I can get them to write it up as $150 purchase $100 donation", I can deduct the $100.
Purchases as part of donations.
I make a $300 donation to PBS, and get apocryphal PBS Tote Bag and Mug, the station's newsletter, a $12 ticket to a PBS-run bluegrass concert, and a Blu-Ray box-set of Downton Abbey.
- IRS has special rules for branded "swag" like the tote bag, T-shirts etc. Those do not count at all against your donation.
- IRS also exempts newsletters that are not "like a professionally edited magazine" and don't have ads.
- Charity-related event tickets, as givebacks, are also exempted. Sporting events are not.
- The fair value of the box-set, $100, must be excluded from my donation.
- Deductible donation $200.