I never get counter-offers if I offer a solid 50% premium on what I ask for. There's no excuse to be stingy with traders, goods are so incredibly easy to produce in non-usable quantities that you can flood every merchant forever with a single dedicated crafter.. Sometimes I give many factors more in goods than I buy, because I want to dump my goods off.
Especially easy when you just dump all your giant cave spider clothing in the depot. I seem to never run out of giant cave spider silk socks taken from kidnappers/invaders..
Also; if you get rejected once you need to offer a significantly better margin the second time. Bottom line. They don't like haggling with you. If they've counter-offered twice and you
aren't offering all the goods you want to trade and aren't lowering what you're trying to buy, you're simply trading wrong. Otherwise quit being so greedy and give them more goods, it's literally impossible not to be flooded in fort wealth.
Yes, I did learn this the hard way as well like you did by having to give them your arm and a leg in order to fulfill the deal, but I just don't bother trying to sneak deals off merchants. I get the goods I want from them, I'm not looking to make 'profit'.
Those puny traders can take all the wealth from my fort they can carry and boast about the good deal they got when they get home, while not knowing all I've done is export my culture en masse while getting their delicious metals.
This will be much more satisfying when I can finally invade the areas around my fort only to find the goods, clothing, weapons, etc were made by my fort.