It might not be your prices-- it might be the trader. If your trader is in the middle of nowhere, especially if it's in an outlaws refuge, people are only going to go to it if they see a deal on the ttc website. If your trader is in a popular town then you're more likely to get people just walking around buying things. Thus, you have to price low and sell popular items if your trader is bad while you can price high with a good trader.

Some people will go to whichever trader has the item/s they want, others will only go to places there's several clustered together or even have their personal favourite shopping area and buy from whichever guild has claimed those traders this week.

TTC prices are generally accurate for what's available at guild traders but you do have to bear in mind the price isn't the whole picture. I often sell spare crafting materials and I'd say pewter is the one I'm most likely to get back as unsold even when I list it below the recommended price, because there's just not that much demand for it. Most people will level jewellery crafting past the point where they need it relatively quickly, and don't bother making low level jewellery to use so there's not as much demand as for other materials, even low level ones.

Low level provisioning recipes, especially the ones required for daily writs and jewellery master writs are the other things I often put up for sale and often get back as unsold. The provisioning recipes drop from daily writs so the supply far exceeds the demand and the jewellery writs are usually more expensive per voucher than ones for other crafts, so they're not worth doing unless you have gold to waste and really want the vouchers.

You also have to bear in mind that people buying in zone chat are likely hoping for a discount compared to guild trader prices, in return for the convenience of being able to sell without having to deal with a trading guild.

Although there's also people who will try to trick and/or intimidate people into selling for less than an item is worth, or buying for more, counting on people using zone chat being newer or less experienced and not knowing the actual value of items. I think it's less common in that game (possibly because addons can give accurate info quickly) but some players would stage whole conversations to trick people who overhear into thinking they're getting a good deal.

Scammer 1: "WTS 50 McGuffins - 10k each"
Scammer 2: "Hey man, don't throw your loot away like that! If you go to [DLC zone] you can sell McGuffins for like 50k each!"
S1: "what really?"
S2: "Since the last update everyone wants McGuffins for [obscure/made-up mechanic or farming method]"
S1: "idk man, I don't have the new dlc yet. I just wanna sell them, I'll take 10k if I can sell them now"
Target player, who saw all this in chat and has access to the DLC: "I'll buy 10"

Later Target Player goes to sell or use the McGuffins and discovers they're worth 1/2 what he paid, if he's lucky. Meanwhile the 2 scammers use some of the money to buy a fresh batch, split the profits and do it again in another starter zone.

Use arkadius trade tools (ATT) add-on, not ttc (other than website) for setting prices. But your add-ons are only as good as your guild sales data. Some guilds will post their att data for multiple 'major' trade guilds to their discord so newer or more 'casual' traders can instantly get up to speed with pricing.

Your greatest allies - and competition - are your fellow guildies. Ask in guild chat, and definitely check what prices are listed in your guild for stuff you intend to sell. If looking for a quicker sale it's ok to undercut a bit but you won't be popular in your guild(s) if you savage the pricing of your guildies regularly.

As others have said, if your guild trader spot is hard or slow to access then you'll have to drop your prices to entice bargain hunters. If you're regularly exceeding quotas in such a smaller trade guild look to upgrade to one with a better location.