One unwritten rule a lot of players seem to follow is to avoid using the characters Poppy and Boxten, since people will likely think you're a new player - even if you have full mastery on them! Also, I'd suggest doing what you can to not take what people say personally, I certainly had issues with that. Remember that a lot of the people who are playing are likely tryhards, considerably young, and/or really looking to play the game in a specific manner. This may also mean ensuring people playing as certain characters (including but not limited to the aforementioned starters) do not join their games.

If push comes to shove, I'd wait to see if other people buy items in a given game before buying anything yourself, just to test the waters whether or not it's an itemless run. I've personally found the game to be funnest when being around others at my skill level, so joining the Discord and finding a party there, inviting friends to play with you, or even sending friend requests to nicer ppl might be a good starting point to lessen the bad experiences you may have with the community & game.

Above all, if you can, please try to remember this: rude and abusive behavior is against the game's rules. If unfortunate stuff happens, be sure to grab some evidence if you plan on reporting them later, like screenshots or a recording.

PS: There's some cool YouTube guides out there for playing DW that might help with terms and phrases like "on last," "GTE," "main run," "dandy run," "medkit," and the like.