AVE high speed train - Spain - buy ticket ahead of time?

The problem is if you don't know your schedule you cannot buy the ticket because the AVE train is seat/train specific. There are discount for advance purchase but then you are locked in with min refund, if any, options. You can work the Spanish train site for some idea. It doesn't have an English option but you should be able to understand it. Or use bahn.de for schedule information.

First, to make the journey by train, you will have to go into Madrid before heading back out to Toledo.
Second, the total travel time from Sevilla-Cordoba (45 min), Cordoba-Madrid (around 2 hrs), and Madrid-Toledo (30 min) is about 3:15. So, yes, it's do-able if you time it properly.
Start by back-timing your trip to Toledo. The second-last train from Madrid is at 7:50 pm, the last train at 9:50 pm. You're arriving and departing from Atocha station in Madrid, so you won't have far to go to transfer trains.
Then, from Cordoba to Madrid, you can link up with the Toledo train with departures from Cordoba at either 5 pm (giving you an hour's leeway) or 7:29 pm (leaving you a 35 minute layover in Madrid.)
So that'll leave you with all sorts of trains to choose from out of Sevilla. Go early and wander around until the sights are open...or sleep in a bit and hit the sights as soon as you get there.
For Sevilla-Cordoba and Cordoba-Madrid, you can get up to 60% off by booking up to 62 days in advance at www.renfe.es. The site is in Spanish, but it's not hard to follow....we can give you details if you need them. There are no discounts for Madrid-Toledo, but book them anyway, to ensure your seats. The tickets will all be downloadable as pdf files so you can print them out, and save them on your computer at home just in case.