Imagine, you want to buy a ticket for the train to Batumi today, for the travel date 9th of September to Batumi... it's impossible, because tickets are sold only 5 days in advance (what for are the computers at the counters ??)
Imagine, you want to buy that ticket tomorrow for the 9th (that's possible because tomorrow you are the max. 5 days in advance) and a return ticket on 12th back to Tbilisi...
it's impossible, because the return ticket you can get only from 7th.
Imagine, you want to buy those tickets not for yourself, but for your friends in Telavi, that's impossible at all, because you need their passports for that (the name on the ticket should be identified by passport) or they just should come the hell themselves to Tbilisi and buy it on their own.