Here's some additional information for those of you that are unfamiliar with batch posters and the how and why they work.
So what IS a batch poster?
A batch poster is an addon that does a live scan of the auction house for every item you have programmed into your groups, calculates a posting price for each item based on the settings for the group it's in, gets you to confirm the posting, and continues to queue up more items.
Batch posting allows you to put any item in your bags, and group them up to be sold at similar prices. The best example of this is glyphs. There's so many glyphs, and it just makes it easier to use a batch poster to do the work for you. You can also set it up to recognize your alts, and add white listed characters to prevent you from undercutting affiliates if you wish.
So how exactly does this work?
Well, to understand how it works is to set it up. I won't go into great detail here, to keep things brief, but you can find many guides for QuickAuctions3 (even though it's gone now) to help you set up the newer clones, ZeroAuctions and APM mentioned in the previous post. Keep in mind these are not the only add ons you can use for batch posting. I am simply more familiar with how QA and clones function.
Open up your addon's config menu, make a group, and select auction settings for the group.
You want to set up how many stacks, and items per stack to post each time, or how many auctions to have up. If you have not been undercut, but have not sold anything, it will not post this item again. If you have not been undercut, but sold on, it will only post 1 extra item. If you have been undercut, it will post 2 stacks of 1 of the item.
You should also specify post time. I set mine to 24 hours, since I post frequently, and it costs the same as 12 hours. 48 hours should be avoided.
Price set up lets you decide how much to undercut competition by, how much the bid should be below your buyout, and threshold is the minimum amount you wish to sell each item in the group for. Fallback is there incase there is no other auctions, or you do not have any auctions up, but the current auction is below your threshold.
There is another tab to allow you to add items to this group. It lists everything in your current character's bags.
Now that we understand the inner workings, we can see how this helps us with auctions. When you click "post" it's going to start scanning and logging on your AH UI each scan for items. When it detects you have been undercut or have no postings it will prompt you to confirm each post, and does the posting for you. This way, you can post hundreds of items very efficiently, and thus allow you to post more items than you normally would.
You can use this for multiple items as you please, but remember every market is different and there may be some experimenting to be done to perfect your methods. To explain this, I will give my current details.
Post cap: 2 stacks of 1
Time: 24 hours
Undercut: bid 50c, b/o 95%
Post cap: 4 stacks of 1
Undercut: 50c, b/o 98%
Post cap 4 stacks of 1
Undercut: 50c, b/o 98%
As you can see, each item needs to be treated differently, so you need to do some research. My TBC and Wrath gems rarely even go close to threshold, but often glyphs will fluctuate between my threshold and fallback. Note that I hardly have any competition for the lower-quality gems, which is why I am able to batch post them like this, as being the only real person in this little niche, the market for them is somewhat stable. This may not be the case on your server.
You can also use this addon to cancel auctions in a similar fashion. It scans your current postings and queues up any undercut auctions to be cancelled, but requires a confirmation for each auction to do so. This can get a bit wearing on your mouse buttons, and I personally prefer not to cancel my auctions unless left with no choice.
So, what items is it a good idea to use a batch poster for?
Anything really. I wouldn't bother using it for armour or weapons, or maybe items in very unstable markets. However, items with constant demand are perfect for use in batch posters. Most items that are required for max level professions and raid consumption or item enhancement are your best bet, because the market for said items is more likely to be stable. So this would be any Borean Leather, Frostweave cloth, Saronite ore, Titanium ore, Cobalt ore, bars of said ores, Eternals, gems, glyphs, herbs, and consumables/ item enhancements used at level 80. Right now would be a bad time to do this with most of these items, since the markets are unstable, but once markets become settled in Cataclysm, it's worth looking into if you frequently post in a certain market.
Batch posters do the grunt work for you, but you still need to use your brains. Batch posters can be a very useful tool in helping save you time, and can be used for a variety of items sold in stabilized, active markets.