Trading in ESO is a full economy in itself, that actually closely mimics the real world economy, so it's a great place to observe how prices fluctuate following significant events. Even if you've never tried selling anything in guild stores, I think everyone should try it for a little bit at least, because it's fun and can earn you a lot of Gs, and also learn a bit about economies on the side!
I wrote a post sometime ago on Inside Trade's forums about how it is theoretically possible to boost your sales from guild stores. I know we're not particularly into trading, but perhaps some of you guys might like to get into trading and earn more, so here are some tips from my experience on how to make the best of it.
1) Install a trading addon:
Most important. These provide lots of valuable information, the most important being current selling prices for items and how popular they are. These addons are also how other traders know what to buy (i.e. underpriced stuff) from other guild stores, and the better the prices are in our store, the higher the chances for sales. I personally use Master Merchant, so I can answer any questions about how to use it.
2) List lots of stuff!
And list them at good prices. The more sales you have, the more things you'll want to list! Even small items like recipes and crafting materials are good to start with, you'll find they'll sell much faster than you expect (especially if your guild store has a trader).
3) Relist stuff:
Items stay in the guild store for 30 days if they haven't been sold, and in that period of time prices can fluctuate considerably. It's a good idea to frequently check your items and relist them if they're overpriced.
4) Item stacks:
These are a bit tricky to handle. In general, I've noticed that processed crafting materials like ingots and sanded wood sell faster in larger stacks (the maximum at 200 is pretty good), whereas items used for improving armour and weapons tend to do better in smaller stacks, depending on their level. For example, the gold items like Dreugh Waxes, Tempering Alloys, and Rosins tend to sell better as individual items, rather than stacks, whereas purple ones may do better in stacks of 10 or so; it all depends on the going price.
Additional point, if you have lower level crafting mats, it's probably a good idea to list these in smaller stacks (50 or so). The reason being that lower level players may not be able to afford larger stacks.
It really pays off to invest a few skill points into your crafting hirelings. They deliver free stuff to your mailbox everyday, and the higher your crafting levels and the more skill points you put in them, the better items you get. Even if you're not a crafter, the materials sell pretty well (especially raw materials), and give you a free source of items to sell without having to farm them. Works even better if you have multiple characters.
6) Don't throw anything away!
When I was a younger player (and didn't know anything about trading), I used to throw away thousands and thousands of crafting materials, bugs and crawlers, armour that I could've deconstructed, and other things just to empty up my bag. So a simple tip, invest in bag upgrades or feed your horseys apples, save everything, and sell.
That's all I have for now; feel free to ask me any questions here or ingame, always happy to help!
Also the new harbourer of the Blood God, who gets kicked and punched everyday.