Yes, you could deduct items you donated to Goodwill even if you had purchased the item from Goodwill.
The value of your donation would typically be the Fair Market Value (FMV) of the item.
However, if you paid less than the FMV for the item, then you must use your original cost of the item as the value of your donation. You can not purchase an item from Goodwill and assign it a higher FMV than what you paid for it and then claim that higher donation amount equal to the FMV.
If the item was priced higher than it should have been and its true FMV is lower, then you can only claim the FMV of the item as the value of your donation.
Take a look at the following TurboTax article for more information:
How do I determine the value of donated items?