ALL donations of items require the same documentation, not just items between $500 and $5000. (If the item donation is more than $5000, there is additional documentation required, but that is added to the basic requirement, not a substitution.)
You must have a written record of when the item was acquired, how much you paid, it's condition when donated, and how you determined the fair market value at time of donation. If you have to estimate or guess when making this record, that's better than nothing. If you are audited, it will be up to the auditor how much weight to assign your proof. If your proof is insufficient, they can deny the tax deduction.
There is no set depreciation percentage, especially for specialty or medical items. Ebay is one good way to determine the value of used personal items but there may be other ways. It also depends on the condition of the item. I happen to own a $1000 travel CPAP machine that was used for about 10 nights, but is currently worthless since the model was recalled for manufacturing defects. You will have to make a diligent effort to determine the value of your specific items.
You can include the sales tax in your cost basis—the amount you paid for the items. However, for items that have lost value, your tax deduction is the present fair market value in current (used) condition, which has nothing to do with what you paid.
