The keeper told DVLA the vehicle was leaving the UK permanently. The registration is closed off at that point.
Sometimes it is an error, for example a car that was exported and later brought back without the record being corrected. Sometimes it is deliberate, because an exported record makes a vehicle harder to trace.
A car with an export marker will be difficult to tax and insure, and re-registering it is a paperwork job that falls to you as the buyer. Ask the seller to resolve it before money changes hands, not after.
Buying an imported car
An import marker is not a warning. Plenty of good cars are imported, and some models are only available that way. It does change your checklist.
The history may be incomplete
Mileage and service records from before the import often do not carry across. A low mileage import can be genuine, or it can be a car whose foreign history simply is not visible. Read the MOT history carefully, since UK testing starts from the import date.
Specification can differ
Imported vehicles may have different equipment, lighting or emissions certification from the UK model. That affects both compliance and value.
Insurance often costs more
Insurers price imports higher, particularly grey imports from outside the EU. Get a quote on the registration before you commit.
Value is usually lower
An import is typically worth less than an identical UK supplied car. Run a car valuation so you are negotiating against a realistic figure.
Check the import paperwork
The V5C should show the vehicle's import status. Ask the seller for the import paperwork and confirm the car was properly registered with DVLA, because a car that was never registered correctly can be refused tax. Check the V5C against the record with a V5C logbook check.