I did think about this carefully before I posted.
There were several issues to my mind that are suspect with you selling this.
Firstly, when you buy a car, you are supplied with a single set of manuals and service book for that particular car. A genuine dealer and owner will keep these safe and have the service record stamped up to show the provenance and integrity of the car as this will be reflected in any valuation and sale price. Selling on a car with original documents obviously enhances any sale.
Secondly, if I lost my service pack, I would go to my dealer and get a duplicate and have them stamp the service book up. This is the correct way to do this. My last car had a duplicate service book and it was legitimately stamped as a DUPLICATE by the supplying dealer - presumably the original was retained by the previous owner for whatever reason. I wouldn't sell on any replacement documents under the guise of being a spare, I would return it to the dealer. I do find it somewhat unethical that you are doing this.
Thirdly, you have actually added the service book to the user manual to enhance the saleability of the item, not just on eBay but also in advertising it here on an enthusiasts GTI forum. I accept that the service book will not have the original vehicle specification sticker but the fact remains that there are unscrupulous people out there who would manipulate this opportunity to their own ends.
The Mk V GTi is now at an age when it is more accessible to younger buyers, who may get caught up in the thrill of owning such a car and who may not question it's authenticity if a service book is supplied with the car. There are also owners who have bought parallel imports from car supermarkets who have not been supplied with the correct documentation. The potential for misrepresentation should be obvious.
In fairness to you, I think you should state how you have acquired the manual as a spare and identify the original car it came from, as well as stating if you are connected to the motor trade and/or a trader in vehicle parts.
This is a responsible VW GTI forum and we all benefit from the information shared.
Cass