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You can merit any post which you can like, even if it is a single word reply but if you are doing this for getting something in return which makes you as untrustworthy person but you won't get banned only will get negative trust ratings from DT members.
DT member has their own judgment...
if your post or the post of the person you want to give merit is high quality then doing a merit reply is not a bad thing. the bad thing is when you give more than 1 merit to posts that use short words like
hi and
how are you!
We had seen 50 merits for a single word reply or one liner reply in the past if it is from a reputed forum member but if the same thing happens with two random members then it may be look shady so its completely subjective.