Absolutely no problem - provided that:
1. If it is a new registration, you have owned the domain for at least 6 months.
2. If you purchased domain on secondary market, time limit doesn't apply.
3. You don't spam. From a selling point of view, you would want to be personalising a sales email as much as possible. That's what I do.
4. I always put something like this as an opening paragraph:
" Please excuse me for contacting you "out of the blue", but I notice that you have the website xxxxxxxxxxxxxxxxxxx.com.au. I own the domain name yyyyyyyyyyyy.com.au and am interested in selling it.
I wondered if you might be interested if the price was right?"
(This is my latest strategy - and believe me I have tested a few. I don't put a price - the "if the price was right" comment tends to get lots more initial replies. Then you have got something to play with!).
5. I always close like this:
" If you are not interested in the domain name, I apologise for bothering you, and you won't hear from me again!".
Hope that helps.
Cheers, Ned