Don’t bother repairing it. Certainly not with a new panel bought from eBay!
It’s 10 years old and done 125k. Buyers will expect some cosmetic problems.
The issue here is not the condition of the car, it’s the OP’s expectation on value. It’s not worth £4k. It’s probably worth £3k, but I’d be surprised if it sells quickly if you advertise it for more than this.
FYI, I just chopped in a 10 year old Golf Mk5 in GT spec but <100k miles. Same rust issue. It’s trade in value was £2k. Add £500 for private sale and 10% premium for the Audi badge and I’d be very surprised to see much more than £2750-£3000 for any A3 of the same age, irrespective of spec, unless it’s low mileage and/or MINT (of which the OP’s is neither).