Assuming there aren't any "special" items like collectibles that are worth more than you paid, then the value of your donation is limited to the fair market values of the items or what you paid for them, whichever is less.
[If you have items that you think are worth more than what you paid, special rules apply. In most cases the deduction is still limited to what you paid. You can sometimes claim the higher value but only if you hold the items for more than one year and then donate them in a specific way.]
You need to report this as an item donation, because the rules for item donations are different than for money donations. But you don't need to list each individual item. You can create a custom listing called something like "estate sale contents" and use the $3000 you paid as both the cost basis and the fair market value.
If audited, you will need some reasonable proof of the donation and the value and/or what you paid. Since you donated the entire lot and will be claiming your purchase price, your receipt from the estate sale, some pictures, and a letter of acknowledgment from the charity should be sufficient. You should not need a detailed inventory of each item with a separate valuation. (Make sure you get an acknowledgement letter that is reasonably specific "thank you for the donation of 2 truckloads from the estate sale of Mrs. Smith" rather than a generic blank form that would be the same form given to someone who dropped off a t-shirt.)
