Is it cheaper or not to purchase tickets for bus and trans and Roma pass before you go or when you get to Rome?And is it cheaper to buy tickets singly?
As with most things, it depends on how long you will be in Rome and how much of the public transit you will use. A one-time metro/bus ticket costs 1 euro. This is good for one metro ride or 75 minutes (I think) on the bus.
The Roma Pass now costs 23 euro. However, you get free admission to 2 sites which can nearly cover the cost of the Pass themselves. We used our Roma Pass for the Colosseum/Palentine Hill/Forum (counts as one) and the Borghese Gallery. We then used the included 3-day metro pass every day, multiple times a day to get around the city.
While it is possible to order the Roma Pass online, we had no difficulty purchasing it from the TI near Track 24 in Termini train station. I'm sure this is cheaper as there is probably at least a shipping & handling fee for preordering it online.
Likewise, if you will be taking trains to other cities in Italy, it is generally recommended to buy your train tickets in Italy rather than before you arrive, unless you are traveling during a very crowded time or on particular trains (such as certain night trains).
I think we bought our Roma passes from the tobacchi shop. It covered the cost of the more expensive Villa Borghese, the Colosseum, 1/2 price at Palatine HIll and 1/2 price at Ostia Antica. Plus, all the bus tickets! Good deal if you are seeing several landmark sites.