can i use an iphone 5s in greece if i buy it from usa

Only an unlocked 5s will work. If you buy a USA phone you will have no warranty for outside of the US.

You can purchase an unlocked device from the Apple store in the US. However, it would not support all bands in Greece, the warranty is not international and all warranty service would have to be accomplished in the US. It is better to purchase a device in the country where you are going to use it for many of these reasons.

From the Apple store only, it would be a GSM phone. The t-mobile phone from the Apple store is supposed to be unlocked to, but make sure you do not purchase from a t-mobile store. Again, it will not work with LTE and may not support all bands in Greece and will not have a warranty in Greece.

You should be able to, however understand the disclaimers. In my opinion, the cost of an unlocked device, and the fact that I would not have any warranty on the device would be a deal breaker for me. I would never purchase as expensive a device as the iPhone without a warranty and one that would not support all of the carrier features in my country. Again, my opinion, you are free to do whatever you wish.