Alright, now you have your two gathering professions. What does this have to do with the Auction House? Well, several things. First of all, level 80s are lazy. Really lazy. Lazy enough that when they switch professions there is no way that they are going to farm up their skill. After a year or longer of playing, many of them have quite a bit of cash laying around, and they're perfectly willing to drop what seems like a large amount of cash on low level items.
Also, on older, more established servers, there aren't too many new players; most of the low level characters running around are usually alts. As a result, they also have large amounts of cash and are less than inclined to farm up skills as opposed to just trying to power level as quickly as they can. Therefore, a lower player actively trying to level up skinning, herbalism, and mining has a valuable resource. Level 80s don't want to spend time on alts gathering items while a lower level is going to do that anyway; so it is not a waste of time for the lower level.
Anyway, enough about the justification, it's time to take it to the Auction House. A general rule of thumb is do not sell incomplete stacks of any item, unless it is extremely rare and / or valuable, like gem stones. Otherwise, no one is going to waste time even looking at a incomplete stack of eighteen Light Leather. (On a side note, as a beginning skinner I've noticed Ruined Leather Scraps almost never sell; best to vendor, smash, or give to a leather working friend. If they do sell it is never for very much, since technically it is a half - stack of Light Leather.) Gems can be the exception to the rule, in general people don't need twenty moss agates. Usually stacks around four or five sell well.
An important thing to do before selling is checking the prices of the other same items for sale - this particular part will deal with production items, like herbs and leather. If prices are wildly fluctuating - like a stack of Peacebloom going for 2g, 5g 10g, 25g, and the odd weirdo that posts it for 100g - I recommend not selling. Look for times when most of the stacks - there will always be very overpriced stacks by ambitious people not really looking at the auction system - an undersell, only by a little. AddOns like Auctioneer help keep track of the average price of what items are going for, though I've yet to find a decent AddOn that consistently shows the overall average.
If a stack of , say, Peacebloom again is going for a bid of 3g 50s and buyout is 5g, the best was to sell it is first make the bid very cheap - in this case, make it go for around 1g. The bid amount should be cheap because then it shows up higher in the list, making it more likely your item will sell sooner. People very rarely just bid on items, especially for crafting items which they normally need right away, and they're more willing to spring for the extra buyout cost because of its instant delivery. The buyout for this example should be something like 4g 84 s. Rounding up, it is also nearly 5g in cost. But, the 84s makes it not quite close enough to 5g that it makes it seem like a better deal. Keeping it in the 90's does not work as well in my opinion, and a loss of less than 10s is negligible, especially if your item sells on a consistent basis.
This is a strategy I've used the entire time, and the only time I've had trouble with it is when I'm attempting to sell rare times like Arthas's Tears, because there is rarely any examples of the item available to compare costs, let alone entire stacks. I pretty much go for what I think is a silly buyout cost to 'test' what people are willing to pay - let's just say inscripters and alchemists must be desperate to get Arthas's Tears.