You could potentially try to use a courier linked to an Escrow service, if this is still even a thing now (you will find lots of escrow services that will scam you). I wouldn't touch with a bargepole for a large purchase, though - I'd used before for maybe £50 in gig tickets...

If the buyer gets a 'man with van' to collect and drive the speakers to them, they may be willing to pay you cash on collection if that reassures the buyer. Not sure it would be much reassurance to a buyer, though - you could be giving them dud speakers, if you were trying to con them. From your point of view, it's hard to count and check a large wad of cash quickly, too.