If it is not the correct part you are entitled to a full refund and seller pays return label.
You do not say the reason but I assume you want to return, if so the reason makes a difference.
If “not as described” seller pays for return, if a change of mind then buyer pays.
NOTE that using “does not fit”is classed as a change of mind, and a private seller does not have to accept change of mind returns, so if wrong size sent it is “Not as described”.