Spamming comments is frowned upon by the community.
Comment spam is currently defined by @steemcleaners as:
10 or more generic comments per day that are not in response to a question posed by the post or comment. “Thanks”, “Great Post!”, “Great Photo!”, “Good”, “Like”, “Nice”, “Wow”, “Cute”, “Follow me”, etc.
Commenting on old posts to hide self up-votes or a voting trail.
Copy/Paste comments that serve no purpose or the purpose is self promotion.
Comments that inform the contributor that they have been up upvoted, resteemed, or flagged when the curator does not actually upvote, resteem, or flag the post.
Unsolicited and off topic referral links.
Continued comment spamming may result in action from the cheetah bot.
More Info: Abuse Guide - 2017.
Does this First Definition mean that Steemit Users are allowed to write (for example) 9 Times a day a comment like „Great“ „Nice“ etc., and then it´s classified as spam? Or will something like „nice“ etc generally be defined as Spam?
Upvoting my own comments on old posts is frowned upon, but selling votes is not? Something wrong with your criteria here
Vote buying is out in the open and anyone who doesn't like it can see it and flag it. Hiding your comments in old posts is just that... hiding them.