Well, I AB daily, for at least 6 hours, I just set it and got to work on real life things. As far as shops, I hit all the food shops, and the less popular book shops. Personally, I dont aim for super valuable items, since I throw everything in my shop for regular sale. I think it is pretty safe when you dont target high end stuff, but stick with the normal. You may think no one wants lower priced stuff, but my shop generates 300k-500k daily from just selling regular stuff priced just under wiz (the shop pricer is great for that too!)
On half price day, I hit all of the food shops, book shops, and supplies shops with all of my shells, netting about 2.5k items per account in the space of 8 hours. Again, I avoid the high target shops like magic, stamps, battle ect. Since everything is half priced, no matter what you are gettign a deal and it is a great way to keep your shops full all month for normal generated sales. One bit of advice, dont sell anything for at least 3 days after half price day since the market is usually flooded with cheap prices, and can mess up your margins.
I do take the time to get on one specific account on half price day to AB only high end shops, with high end settings, I do that for 12 hours with a on for 35 minutes off for 35 minute type schedule. That account ususally gets 150-200 items, at least half unbuyable on half price day that I sell slowly all month.