Yes every seeds has a few fixed positions they can spawn.
But globaly/generally its like random where these spots are, even if allways the same spot on a specific seed.

@ to
3 options you have

1: The legit way by explore a circle in arround 20-25% worldrange, Check every Dark forest on that circle Arround 90%+ of the seeds have a traderspot on that circle

2
Name your seed and we can give you a hint where the trader is

3: Check it out yourself
https://valheim-map.world