Differences between BfA and Classic
BfA and Classic have different mechanics for calculating deposit costs which means that the optimal stack sizes and optimal strategies are different between the two versions.
Some things are similar between both versions, and that is the basic functionality. You pay the deposit cost when you post an auction, and you get it back if the auction sells. Otherwise it is lost. The cost is based on the auction duration with a higher cost for higher durations. For most items in both versions the vendor sell price is the basis for the deposit cost, with one notable exception in BfA.
Classic deposit costs all scale with the vendor sale price. So posting one stack of 20 or 20 stacks of 1 for the same material will cost you the exact same amount of gold. Duration is the only thing that will change it. This might indicate that you should mostly post in small stacks, as it increases the chance you sell at least some of the items.
The main thing you want to avoid in classic is to post items for sale that are just selling for barely more than the vendor price. Unless the price you can expect to sell it for is at least 5 times the vendor sell price you should probably not post it (some fast selling materials can be exempt, but even then it should be 3 times vendor sell to have enough margin for error). The lower the vendor cost, the cheaper it is to post. For some markets this matters and you can use it. For others it doesn’t help. You can’t change the vendor price of crafted goods. You can choose to flip lower vendor cost materials though.
For the most part deposit costs work exactly the same as in classic, so the logic is exactly the same. With one important exception. Materials in BfA have a deposit cost that is based on 20% of the per item price. This means that posting 200 stacks of 1 will be 200 times more expensive than posting 1 stack of 200.
The implication is simple. Post materials in larger stack sizes. 20 or bigger is more than OK and will give you a pretty good margin of error for material flipping.
When I’m posting Pets, I’d like to post 2-5 at a time, but with the new system you have to create a new listing for each.