The availability of the customary tax benefits (depreciation expense, Section 179 first-year expensing, etc.) are not dependent on your having a receipt.
In the unlikely event that you are challenged on the actual amount you spent on the assets, you've got your cancelled check.
If I remember correctly, used assets don't qualify for the first-year 'bonus' depreciation, but do qualify for both regular depreciation and the Sec 179 first-year expensing. On this point, though, I'm just speaking from memory, and you should verify this with a tax advisor, or through your own research.
Also, the foregoing deals solely with federal income tax law. Some states require depreciation methods that differ from federal rules (not recognizing Sec 179, e.g.), so check with the Wisconsin rules before you prepare your state return.
...it was early and I was full of no coffee...