Is it appropriate to send a PM to someone who request help in technical forum, telling him I can help him for a fee?
(after my advice on the forum did not help him)

I would have thought that that would be fine - I would like someone to offer me help if I needed it. Everyone can chose to refuse paid help if they wish to.

I don't see anything wrong with that, so long as you don't make a post in the thread asking them to check their PM box. Be warned though, it might be seen as spam by the recipient.

ok, thanks for the replies and the heads up ':)'

personally, i usually consider it spam. if i were looking to pay for service, i would have sought them out or asked for it. if i am asking for help, i am trying to see if i can do it without having to hire someone.

kind of depends on how the post is worded IMHO.

It would be grey to say the least. When I post, I'm generally not looking, nor would I want, someone to contact me offering their services. If I had wanted that, I'd have posted in the Job Offers Forum, which some do. In general, I'd refrain from sending commercial messages to people who don't request commercial services. If we get a report of PM spam, we have to act on it, and users are generally warned once, and then banned the second time.

Point well taken.