My understanding of this is as follows:
The Talmud here is citing a Mishnah in Ta'anit (12b) which discusses the progression of fasts when there is a lack of rain:
עברו אלו ולא נענו ממעטין במשא ומתן בבנין ובנטיעה באירוסין ובנישואין ובשאילת שלום בין אדם לחבירו כבני אדם הנזופין למקום
IF THESE PASSED AND THERE WAS [STILL] NO ANSWER TO THEIR
PRAYERS THEN BUSINESS IS RESTRICTED AS ALSO IS BUILDING, PLANTING,
BETROTHAL AND MARRIAGE; AND MEN GREET ONE ANOTHER AS PEOPLE
LABOURING UNDER DIVINE DISPLEASURE. (Soncino translation; capitals in original)
In its interpretation of the Mishnah, the Talmud there (14b) cites a Beraita to define the parameters of the ban on business and planting:
תנא בבנין בנין של שמחה נטיעה נטיעה של שמחה אי זהו בנין של שמחה זה הבונה בית חתנות לבנו אי זו היא נטיעה של שמחה זה הנוטע אבוורנקי של מלכים
It has been taught:
By BUILDING [is to be understood] building for joyous purposes, and by PLANTING planting for
joyous purposes. What is ‘building’ for joyous purposes? — Building a house for the marriage-feast
of one's own son. What is ‘planting’ for joyous purposes? When one erects a royal banqueting hall. (Soncino translation)
Tosafot back in Megillah picks up on the fact that though the Mishnah forbids business, building, and planting, the Beraita only defines the parameters for building and planting. The Beraita does not tell us anything about the parameters of the ban on business.
To this, Tosafot begins by asserting that the ban on business mirrors the ban on building and planting, meaning that only "joyous" business is forbidden just as only joyous building and joyous planting is forbidden. Tosafot then seeks to prove this assertion. The proof is that if in fact all business was forbidden (and not just joyous business) then building itself would be forbidden, because building is the paradigmatic example of business (אין משא ומתן גדולה מזה). But since the Beraita explicitly stated that only joyous building is forbidden, all other building is perforce permitted. Since all other building is necessarily business it is thus demonstrable that regular business is permitted (as long as it is not joyous).
Tosafot is not giving us advice about what line of business to go into; Tosafot is simply trying to prove that regular business is permitted even when the Mishnah states simply that business is forbidden.