Generally we say that the same "social contracts" or policies that guide our offline behavior also apply online. Do you allow people to sell girl scout cookies in your office? That should be a good indicator of whether or not you allow them to sell online.
The same way that we "allow" people to exchange information about their personal lives at the water cooler, in the cafeteria, or before or after meetings, we allow them to do the same online. It builds relationships and ultimately has a positive effect on productivity. However, it is understood that the main purpose of these events and venues is not for chit chat or profit, and that goes for online conversations as well. To clarify this in our terms and conditions, we remind people of the purpose of the community and that they should consider that before posting. That should give you something to fall back on in case you find anyone is abusing the feature.
I'd start by finding out what your solicitation policy and practices are now. We are following our corporate non-solicitation policy in regards to employee to employee solicitation. Our employees aren't supposed to sell girl scout cookies, fundraiser for personal causes, promote other commercial businesses, etc. inside the office, so they are also prohibited from the doing the same in our Jive site. But that's our company, and a different organization could have a different point of view
We have a "Company Store" webpage on our current intranet where we promote any employee discounts offered by vendors or local businesses. That site also prohibits businesses owned by employees or close family members (just to avoid any appearance of impropriety). We wiill migrate the Company Store to Jive as part of our overall intranet migration.