@drewjza70,
The reason may be due to a few factors, such as the sale of vehicles/motors etc being dealt with on eBay under separate policy. I know that Vehicle Parts & Accessories are not under Cars, Bikes and Boats, but in the US (and I am pretty sure in Australia initially) it's all under eBay Motors. The guidelines for listing and selling vehicle parts in the US are much more restricted than they are here!
Wherever it seems that there are additional / external laws that may impact on the sale or purchase of particular items, it appears that eBay will pop those sorts of goods into the Excluded items category for the purpose of eBay coupons/redemption offers as there is a potential for it all to be come very messy.
It's clearly no coincidence that one sees similar restrictions impacting required forms of payment that sellers must offer, the exclusion of some items from eBay's Money Back Guarantee, the issue of non-binding bids for certain categories, and the separate Motors policy on eBay.
From the Accepted payments policy:
โ 
Restricted
Although they may choose to do so, sellers are not required to offer PayPal or merchant credit card facilities for listings in any of the following categories on eBay.com.au:
- Cars
- Motorcycles
- Aircraft
- Boats
- Caravans
- Trailers
- Trucks (commercials)
- Services
- Real Estate
- Businesses for saleโ
From eBay Money Back Guarantee policy:
โCertain items are not covered by eBay Money Back Guarantee:
- Vehicles, Real Estate, Websites & Business for Sale, Classified Ads and services, Tickets
- Digital Goods and Intangibles, with the exception of digital gift cardsโ
From Non-binding bids policy:
โIn our real estate and businesses categories, bids are considered non-binding.
[...]
Certain types of items cannot be sold through the auction or fixed price format on our site. Therefore, we have a separate policy for those types of items where placing a bid neither constitutes a legally binding contract or obligates the seller to sell the item. The high bidder is not obligated to purchase the item either. If you list these types of items on eBay, the listing cannot result in a binding contract to buy and sell the item, but is merely a means of introducing interested potential sellers to interested potential buyers.โ
From the Motors policy:
โCars and other motor vehicles are typically high-value items, so we have some additional guidelines in place to make these transactions go smoothly and to protect our buyers and sellers.
Bidding on a motor vehicle is slightly different from bidding on other items.
Under the terms of such a bid, you are expressly consenting to share your information with your seller under certain circumstances and consenting to the seller contacting you about your offer on the vehicle or similar vehicles for sale.โ
From the Digitally delivered goods policy:
โWe consider digitally delivered goods to be information or content that you can download from a website or receive by email.โ
(Because of the ease with which such goods can be copied, eBay obviously don't cover these in their MBG.)
From Selling with Classified ads:
โWhen you list an item through an eBay Classified Ad, you state a price, the buyer contacts you, and together you finalise the transaction outside of the eBay platform.โ
(Again, not covered by MBG.)
I actually don't see any reason why vehicle parts should be excluded in Australia from eBay's discount coupons/redemption codes as it's very probably a residue or vestige of the handling of related issues in the US (statewide rather than national, I think) rather than Australian issues. However, eBay are unlikely to change their minds and exclude this exclusion - probably for the sake of simplicity.
