I would gladly pay a substantial monthly subscription for the parts compatibility feature as an add-on.
Adding a new API has to be labor intensive, and if it's to be an offline tool it would require the ability to update in the same way store categories do now, but with a much larger database of compatibility information. I think that most motors sellers would be happy with the only having the option to manually input vehicles to a user generated compatibility list that we could save in our templates, if that is easier to implement than integrating with eBay's compatiblity catalog, thier catalog does not work all that well as it is, and I know that I have to manually enter each vehicle.
As it stands I have a few options to work around this. I sell used motorcycle parts. Sorry for the wall of text, and please know I'm not trying to "advertise" other options, I'm just trying to explain the options for motors sellers and how we don't have good listing tool options, and how on a Mac they're even worse.
I can list with Garagesale, then use the Froo Bulk revision tool for free via eBay applications, but this takes an extra 5 minutes a listing and doesn't offer the same available vehicles (sometimes) like manually inputing through the eBay, so I lose customers because they don't know my part fits and it's a very clunky way to list. I lose time.
I can list with Turbo lister in parallels or bootcamp, but Turbo lister is a buggy pile of doo doo and it takes just about as long to use it as it does to use the "sell" link on eBay. My listings look plain and unprofessional. I lose time.
I can list with Auctiva, using their online tool, subject to internet connections, browser crashes, and their questionable business practices, including injecting their adverts into my listings. (I tried them for about 3 days, no way, never again) I lost a lot of time.
I can pay for a subscription to MotoLister and use it through Windows. It's either $50 a month, or $0.10 a listing (max $150 a month). Now moto lister has some cool features, including pricing help and their own catalog for cross reference help. BUT, it's not a perfect solution it has significant flaws, $50 is fairly large monthly investment, the tool is online and subject to those shortcomings, I have to use Windows, adding a layer of work to picture files and record keeping, plus it's only for powersports, and I do have an automotive section in my store, leaving me to choose from the above options. I think in the current situation, I'll use motolister for a few $0.10 listings that have 300-1000 vehicle fitment options, those listings that take the longest to add compatibility to after creating.
So you see, the options suck, and the best option already has a monthly subscription cost. If Garagesale could implement this feature and charge in the realm of $25 a month, and maybe optional per listing charges for smaller volume sellers, I think you'd have a winner. ESPECIALLY, if you could implement into your mobile listing tool at the same time (I never list using mobile, but I know it's a growing segment and as a per listing charge, I think it would fit well). Having all the listing solutions wrapped into one application and solely on our Macintosh...YAY. I don't know how many current users would be willing to invest in a subscription or pay per listing to add this feature, but I know once word grew that GarageSale had Parts Compatibility, that new users would come, and eBay would most likely add GS to the very short list of tools that offer Compatibility.