How should I post screenshoots of messages when the user blocked me

You can't get them back - the raw steam chat screenshots so its important that you get them along the way of conversation.
Also keep in mind to never discuss steam trades outside of steam chat

These links are also available to anyone at any time - they are also provided before you click to submit any report.

https://forums.steamrep.com/threads/report-submission-fundamentals.169802/
https://forums.steamrep.com/threads/screenshot-guide-requirements.157969/

They are located at the top of this Reports area
https://forums.steamrep.com/forums/report/

It is fine to ask these questions, however please do not reference any reports you may have submitted as that is against GF FAQ
https://forums.steamrep.com/threads/general-discussion-faq.132723/
#3

Thanks! - keep in mind you can report to Valve also if you were not able to do so here.

Those logs are inconsistent for starters vs what a raw chat box will show. Basically if you both are messaging quickly within the same 60 seconds messages won't be in order. There are other reasons that I won't really get into to avoid someone getting ideas on how to circumvent our methods of validating chat logs but that is the main reason. They can be used but not an acceptable replacement - IE: if you are showing me logs from the raw chat boxes that are too cropped and barely acceptable we may ask for the others to compare what you have to lock down 100% that was the conversation as it was held.

Sorry - you can again report them to Valve instead but in the future take screenshots as you go along and save them - you can always delete later.

And again pay close attention to that FAQ #3 I gave you on the first reply - referencing past reports that were not accepted are not what we allow here but you can question policy in general.
Thanks!

You can see past steam chat messages by visiting https://help.steampowered.com/en/accountdata/GetFriendMessagesLog