i have my doubts if that will be doable at all, since first of all ratingbuster looks at the overal stats on the item you have equipped, with gems and enchants on it, so even though an enchanted + wel socketed item is maybe a tiny bit better than a not enchanted piece of gear, this piece of gear will simply not be shown as an upgrade by the mod you want to make.
secondly, if it is doable, then a manual search and compare would go a lot quicker than letting an addon compare each and every piece of gear on AH just to see if its an upgrade or not, this would i think consume a lot of memory and not to mention time for the scans. While if you do it manually, you can have it done within a minute.
Besides, on high populated realms, the gear on an ah is bought and added every few seconds and this would mean that the addon, if you want it as accurate with the list as possible, also needs to scan if the gear is still there, and if there's new gear added in the time it has scanned, meaning an infinite scanning loop which never gives a good result.
third, i think if it was even remotely possible, the guys from auctioneer would've already implemented such a feature into their package
i'm not a coder / dev myself, but thinking thoroughly about the actions the mod has to do, and the fair share of code that is required to do so, and the 100% flawless integration with the AH coding of blizz, makes it a bit too hard to actually make such an addon.
but if any developer can come up with an addon like this, i would love to alpha and beta test it and eventually use it as well
Greetz,
CM