I am waiting for the right time to buy a ticket from Istanbul to Ankara on this website: https://ebilet.tcddtasimacilik.gov.tr/view/eybis/tnmGenel/tcddWebContent.jsf
The problem is that I have to choose time, which isn't flexible for me, so can I simply buy the ticket when I am at Pendik station? or would the prices and seats differ a lot? If I buy online, can I change the time I can use the ticket?
You can purchase a ticket at Pendik station between 5:30AM and 10:30PM. Post offices (PTT) sell tickets as well, but only the branches listed here.
If the day of travel is at the weekend, it is difficult to find a ticket on the same day. You are more likely to find one on other days.
There is no dynamic pricing, so the prices never change.
The most important part is that all tickets (even if standard or flexible) are exchangeable and convertable to open tickets. Flexible tickets additionally have refund option.
You can buy a ticket on the day if travel, however, on some trains you run the risk that all seats will have been sold for that train. In which case you will not be able to purchase a ticket. The man in seat 61 says: "It's easy to buy tickets at the station when you get to Turkey. Most major stations have a computerised ticketing & reservation system, so can book any journey in Turkey. It's not usually difficult to get seats or berths on the day of travel or a day or two before, although sleepers can sometimes get fully-booked." https://www.seat61.com/Turkey2.htm
You may also be able to purchase train tickets from a travel agent if it's more convenient for you. Some examples are listed at the same site above.
