No road touches this parcel but the seller keeps saying there's a dirt track

There are two separate things here, and the words people use blur them. Physical access means a truck can get in. Legal access means the parcel has a recorded right to cross whatever land sits between it and a public road, usually a recorded easement, sometimes road frontage it just doesn't look like it has on GIS. A dirt track everyone uses is physical access with no proof of legal access.

Prescriptive easement is a real doctrine, but it isn't automatic and it isn't something you can hand a buyer as a fact. In most states it means someone has to establish it, often in court, based on how long and how openly the route was used, and the requirements differ state to state. That's a question for a real estate attorney in that county, not something you resolve from GIS.

So the practical path: get it under contract with an inspection or diligence period, order the title commitment, and see whether an easement appears in the record. Title work on a cheap rural parcel usually runs a few hundred dollars. If there's no easement of record, you're not necessarily done. Sometimes the adjoining owner will grant one, and sometimes the adjoining owner is your best buyer, because a landlocked parcel next door is worth real money to them and nobody else.

One thing worth planning for now rather than later: whatever you learn about access, you tell your end buyer in writing. A builder who finds out at their own title search that there's no way in will walk, and word travels in small land markets. Price it as no-legal-access land and market it honestly, or don't take it.