I live in Tier 4 and am no more likely to spread Covid than someone in Tier 1 or 2. We spend most of our time alone, rarely going out and never where there are crowds.
I would do what someone else suggested - show them around, keep your distance (maybe stay outside the room while they go in and look around), have a sanitizing session afterwards, insist on masks, supply them with new gloves if they don't have any so you won't worry too much about what they touch.
Get the estate agent to ensure that they are in a position to buy and are seriously interested before taking the risk - there are still people taking every opportunity to have a day out.
Good luck with getting a sale