Should I only accept cash when selling a car privately?

I thought electronic transfers of money occur instantaneously? You both go to your bank and you execute the electronic transfer. The seller would check by telephone or internet banking that the funds are in his/her account and then the car is yours.

Im in Ireland, seller is in Glasgow and bank transfer is happening on Friday afternoon. Also my bank confirmed it would take 2/3 days to clear and autotrader have mentioned this also on their website.