Punki,
I am not sure how to solve this, this is why I posted here so that Randy could give us his opinion.
I would just guess that posting "I'd like to exchange 4 HK-49 for 1 SWU" is against UA rules (whether we should worry about them seems to be the real issue).
What about posting simply that you have 1 SWU to give away? Now, if you give your SWU away to someone who never gives anything back, you've obviously lost. But if you give to anyone in a group known to give as much as they take, isn't there some chance that at some point you'll come close to being even?
... I wish I had SWUs to give away

I always seem to use them up, and I would be more than pleased if the rules allowed me to buy them!