i'm wondering is there a rule saying that we cannot promote other forums by posting in our post?

e.g: forum XXX have this and that, u guys should go check it out <--something like that...

p/s: i've tried searching the forum, but no result come out...

Doubt you can .

It depends on the context. Blatant promotion is prohibited, but if your primary motive in posting it is for some other purpose then its fine.

Moderators discretion applies

The example you gave in your post is most probably not allowed.

so if like the situation is like the following:

some forummer asking for some guitars at rm2k range, and so to happen that i saw one thread in some other forum posting saying there's a guitar for sell at that price...

so is it ok for me to post the link in the forum for the forummer to go and see the post, which is in another forum?

just wanna make sure, don't want to break any rules while posting here...

Yeah, that's acceptable IMO, you're not promoting the forum link, but instead your giving him the link for the source.