As an omnicrafter myself, I'll offer some tips that would make your wares marketable to me, and likely other omnicrafters as well.

If a gatherable item has a shop selling price, take it off of your list. Trust me, we know what these are and where to go. Examples: Iron ore, Cinnabar, Ash log

Take advantage of our laziness: What this means is to sell gatherables that require high quantity, are hidden, or on timed nodes. You could gather and sell things like silver ore and electrum ore, but honestly we can send our retainers after those and they'll bring back more than plenty in a couple of trips, or we can gather them ourselves if we don't want to wait. Not to say they are not still marketable, but high quantity stacks will take longer to sell. Items like Birch logs (hidden), Titanium ore (hidden), Zelkova logs (timed node), Adamantite ore (timed node), Molybdenum ore (high qty per synth), Persimmon logs (high qty per synth) are examples of items we go through quite quickly. Retainers can't keep up, and are time consuming to gather. These require more effort/time, but will sell at a higher rate, and bigger stacks.

Forget about HQ and take advantage of our OCD: Unless it's endgame gatherables that we actually need HQ due to difficulty of crafting the recipe such as Silvergrace ore, don't waste your time with these. I honestly don't care if its NQ as I can easily craft HQ items from NQ materials if need be. Sell the HQs you acquire in low quantity and typically not the quantity of the multiples we're after. If lumber requires x3 or x4, don't sell HQ in these quantities. Sell them in in quantities that are prime numbers and are/or not multiples of a recipe (1,7,11) as I will often need these to make my stacks divisible. You can also sell these at high prices. You'll be surprised what OCD omnicrafters will pay to make their stacks divisible. Within reason of course. I'm not going to buy a single HQ log for 10k.

Don't undercut and dispatch your retainers to multiple cities: Take a look at the sale history on the MB. Does your wares sell multiple times per day, and everyday? If they do there isn't much reason to undercut it. Your competitors are already going to take care of this for you, so don't sell yourself short. Many buyers and sellers are not aware of tax, and if the cost of tax is greater than the cost of a port, I will port to the city it is being sold from to buy it. If you undercut by a huge margin, I may very well not buy your wares out of spite.

Send out those retainers and make sure they are geared up. The more you send them out, the more they bring back. The better they are geared, the more they will bring back per venture, up to a cap. Get them capped asap. This also helps with the HQ advice I told you about earlier.

I can offer more advice, but this is getting rather long. Try some of these out and see how they work for you. It's not too difficult to make a mil per day through gathering if you apply yourself correctly.