I guess the sales tax applies only if the buyer is in the same State right ?
And then it also would depend on if the taxing authority considers domains as a service or product, and if they tax services. Here in FL, services are (usually) not taxable, but products are. Services are taxable when sold with a product however, and certain other special service taxes are levied on particular industries. And then, you also need to consider the moratorium on internet taxes, and whether this applies or not.
They may not even be able to give a sure answer, but the best thing to do would be to ask a local tax attorney. Not just an accountant, but an attorney. And make them put it in writing so you can defer to them if you have problems.