train tickets

Book the long distance trains asap as the prices go up the longer you wait
In July currently it looks to be 1/2 the price cheaper to fly from Barcelona to Seville in comparison to taking the train.

I use trainline.eu and have used renfe but not loco2. Travelers on the forum seem to have to most trouble using renfe.

I used loco2 for my Spain trip and bought all tickets in advance, Madrid-Cordoba, Cordoba-Granada, and Seville-Madrid. I found their site to be very user friendly. They also gave me great customer service when I needed to make a change to the ticket from Cordoba to Granada. They emailed me back within two hours and completely made my change without fee. I tried to use Renfe but found it just too difficult to use, and I admit that it could have been user error on my part! They are all Renfe tickets no matter what website you use, and if there were any additional fees for using the third party loco2 site, I either didn't notice or they were too small to really matter. Either way, I hope you have fun on your trip!

I'm not sure if that is you real itinerary but I would suggest that you do
Madrid to Seville (train)
Seville to Granada (train)
Granada to Barcelona (fly)
Barcelona to Madrid (fly).

I think loco2 will charge you an additional fee over the price of tickets.
Many day trips you can do by bus (ALSA) so make sure you look into it.

I agree with Nestor about looking at flying between Barcelona and Granada, because that cuts the number of (comparatively slow) ground-transportation legs in and out of Granada from two to one. However, there is no reason to fly between Barcelona and Madrid since the train can take as litle as 2-1/2 hours.