To demonstrate more directly this is just a coincidence:
Pronunciation of 意 is derived from Early Middle Chinese [ʔɨh], which naturally develops into yì in Mandarin.
Pronunciation of 易 is derived from Early Middle Chinese [jiəkʲ], which also naturally develops into yì in Mandarin.
The word 生意 occurs in texts since at least the 3rd century (originally in the meaning "vitality, livelihood," with "work" or "business" shades occurring since around 16th century).
The word 交易 occurs very frequently in the classical texts such as the Chronicles of the Historian and even before, in the Yi Jing, already in the modern meaning.
Thus, both words appeared long before the pronunciations of the two components coincided as yì in Mandarin.
(Data on occurences taken from the 漢語大詞典, which is not always able to pinpoint the actual first appearance of a word - but at least allows to give an upper estimate to it. I am sure that 生意 is even earlier.)