A court likely would find not preempted a State’s general sales tax that applies widely with no particular hook to employee benefits.
For one exposition of that idea, read https://cases.justia.com/federal/appellate-courts/ca6/12-2264/12-2264-2016-07-01.pdf?ts=1467388843.
Unless a particular precedent of the U.S. Supreme Court (or at least the Federal circuit in which the matter would litigate) fits and controls your client’s situation, there usually are arguments that point in another direction. And even if a precedent controls, one might argue that the precedent is incorrect.
If a big-enough amount of the would-be tax calls for the analysis, the plan’s fiduciary would—perhaps with your law firm’s advice—estimate the probability-discounted refund the plan could win against the estimated expenses of litigating the plan’s refund claim.
Even if the State’s law provides an award on a showing that the government’s position was not merely incorrect but also lacked a good-faith argument, a fiduciary likely should run a cost-benefit analysis assuming no order for the government to reimburse any portion of the taxpayer’s attorneys’ fees.
Even if the circumstances otherwise call for cost-benefit analysis about whether to assert the non-application of the tax, your client might want first your advice about whether the sales tax is imposed on the seller, or on the buyer (with a requirement for the seller to collect and pay over the tax). If the sales tax is imposed on the buyer, there might be little or nothing for that person to litigate.