There is a lot of those threads with a person trying sell something and writing min price: wtf high price something. Am I actually allowed to bid something lower in those threads? Or should I just ignore them and maybe refresh after a day or so if they've maybe lowered it?

Same with auctions. Can I ask them to lower min bid a bit or no (if no bids for some time)?

It's there so they don't get any offers that are under what they want.
And also it stops them getting stupid offers.

The minimum bid is what that person is willing to go minimum on that item. Its also the bid that starts the auction. You can ask them to change it via pm... Asking in the thread is likely to end up as an infraction for a useless post. The other option is to let the auction go and don't mind it... You don't HAVE to join the auction. Just wait for the item to come around again in the future.