Do you have more specifics on the problem and the specific part needed?
The 2011 is now considered vintage, and they have a lower resale value because they suffer from a serious design defect with the GPU, which can render the computer inoperable. Depending upon the issue, you may need to remove the logic board from the computer - it's doable, but a slow process, and one where something can easily go wrong. Further, since the computer is now vintage, Apple might not sell you the part depending on what it is. So then you would have to find the part from a third party seller, and inexpensive used logic boards start at around $450, which is more than the value of the computer in its current condition (and new is often much, much higher.)