Just curious would LE have access to account passwords across sites they've seized as well as by other mean e.g through requests to reddit, btc companies etc. and could they use this information to ID you? I know I'm thinking in the worst caste scenario but it was just a thought i had recently with all these new markets appearing and disappearing. I myself try to change the passwords i use every now and again and avoid using darknet p/words for clearnet stuff
It depends. If the site is coded properly, then all passwords will be hashed or run through a KDF, and only the garbled "password" is stored. However, if the site was left running after seizure (as was the case in the pedo forum bust last year), then plaintext passwords could be intercepted as they were submitted.
You should never, ever, EVER re-use passwords or passphrases, period, but especially not between clearnet and darknet activities. Tails comes with KeePass for exactly this purpose - use it. It will generate and store very long, very complex passphrases that are unique for each site you use. I'd also recommend using it in your clearnet life as well - it's a very useful program and gives an enormous boost to your online security, no matter what you're doing.
Don't use any password managers that are cloud-based, like LastPass or 1Password, and make sure that what you're using is Free, Libre Open-Source Software.