You can get the likely value now as it is set by the authorities these days rather than the buyer and seller adding on about 50p to the value but I can tell you if you bought five years ago the value declared then will probably be minimal and the set value in two days will be fairly reflective of the true value. Not knowing the details, even based on a £40k apartment of 5 years ownership you might have something like 20,000 lira (£7,000) on your tapu and the new one might be close to full value (say it it 110,000 lira) £40,000. 40 - 7 = 33, 25% of 33 = a tax bill due to you of £8,250!!
I doubt he'll pay now and wait, get a deposit of serious value and a contract drawn up by a solicitor saying the title deed will transfer to the buyer on <date> at which time he will pay the balance. The solicitor can hold the deposit if it makes him feel better.