Is there a subdivision plan for your development? If so, often there is a clause that prohibits commercial uses, or limits it to cottage industries. The restrictions might appear on the subdivision map or in a separate covenant of restrictions.
An alternative would be to see if you can amend your bylaws. It may be too late to amend them to prohibit commercial enterprises, but you might be able to put in a clause saying that any costs to repair damage to the road above and beyond the normal wear and tear caused by residential use will be billed to the landowner responsible for that damage.
Or, when your road association votes on their assessment formula, they could vote to have a different level of assessment for commercial uses of the road. But be careful not to go overboard on that one. I know of one association that decided when someone wanted to put in a Bread and Breakfast that they could assess that property to pay most of the entire cost of maintenance of the road. As might be expected, that did not go over well!
There should be a reasonable justification for having different assessment levels - for example, when an association charges less for all seasonal residences than for all year round residences, or charges less for undeveloped lots than for developed lots. You would have to have a formula that is "fair and equitable." The question is, how would you determine that? Would it be based on the amount of traffic generated by the business compared to residential use? That could be difficult to calculate. Besides, having a "commercial" level in your assessment formula could suggest to other owners that your road is open to commercial use, so it could backfire.
As far as the liability goes, talk to your insurer. Maybe they have some suggestions. And keep talking to the potential buyer and expressing your concerns. If your insurer gives you a figure for increased coverage, let the buyer know that is one of your concerns. You might also be able to somewhat limit your liability by posting signs saying, "Private road - pass at your own risk."
One last thought - if this happens to be one of those "private roads" that is also a "public easement," there are a couple of statutes that can provide some relief if someone damages the road. But they only apply if it's a public easement.