To live, in Korean, is 살다. To buy is 사다. There’s only that ㄹ 받침 (final consonant) that makes these two different.
Now, the thing that concerns me with these two words is when they are used in the noun-modifying form. 살다’s verb stem 살 loses the 받침 ㄹ and then comes the -는 to make the noun-modifying form. Hence:
살 – ㄹ = 사 + 는 = 사는
Now, with the verb 사다, you just drop the dictionary form ending 다 and add the -는:
사 + 는 = 사는
I think this is where the entire sentence needs to be taken into consideration to understand the meaning of the word. Take, for example,
서울에서 사는 사람은 민지예요.
백화점에서 옷을 사는 사람은 수미예요.
Two different sentences with different meanings…which may or may not confuse anyone who’s studying 한국어.
EDIT [2011.05.06]: I thought about it a few weeks earlier, and, correct me if I’m wrong, but I think people don’t actually use 사는 all that often in the first sentence–they’ll probably use 살고 있는 instead, which makes it more…temporary, I guess?