I am trying to say: "Shall I buy it?" Which would be better?
私は買いましょうか? or 私が買いましょうか?
I am trying to say: "Shall I buy it?" Which would be better?
私は買いましょうか? or 私が買いましょうか?
Assuming you mean "Shall I buy it?" in the sense of offering to buy something (i.e. "Do you want me to buy it?" or "Shall I be the one to buy it?") then 私が買いましょうか? is best.
If "Shall I buy it?" is wondering to oneself "Hmm... shall I buy it?" then neither. That would need a completely different phrase to the two offered in the OP.
Instead of answering which is better, I would like to write some opinion. If in the situation here,
Shall I buy it?
"I" is not that important (the person you are talking to, knows it is about "you buy or not", not "who to buy it"), I suggest 買いましょうか{かいましょうか} / 買おうか{かおうか} / 買っちゃうか{かっちゃうか} ... which means you wish to have some feedback / you are considering buy or not youself (but saying.. hah)
Apart from "shall I buy it", you are actually asking the difference between は and が. Let's keep discussing if you still have problems :)
一緒に頑張りましょう (look here, you don't have to say 私たちが{わたしたちが}一緒に{いっしょに}頑張りましょう{がんばりましょう})