Trade is not 'सौदा'
I think 'Trade' means 'धंधा', 'लेन-देन', 'क्रय-विक्रय' and 'व्यापार'('Business' is more appropriate for it) in Hindi. This differs in meaning from 'सौदा'( deal, bargain etc.). But Duolingo is translating Trade as 'सौदा'. I consulted many online sources and books. Not a single source translates 'Trade' as 'सौदा'. There is a small difference in meaning of both the words.
Jattboy, the Shabdkosh dictionary does show "trade" as a translation for "सौदा". (See here). And I checked out a couple of other dictionaries too, and they all show "trade" as a translation for "सौदा". व्यवसाय, कारोबार, लेन-देन and व्यापार are also accepted as alternatives for "trade" though.
Literally we do not use 'trade' as a substitute for 'deal or सौदा'. It just does not feel right.
a lot of words fade away in books and society slowly starts using different words for the same thing. सौदा is not very popular as meaning 'trade' today but still some people use it. remember that the devanagari script was almost completely changed about 60ish years ago(we use a newer version not long ago अ , एं were written very differently) and alternate spellings for words keep popping up .
you are right that why we always say "सौदा पक्का हो गया" for "the deal is done" and never ask a person "आप किस सौदे में है" for "what business are you in"
but people also ask "आप किस चीज़ का सौदा करते हैं ?" :) language is very dynamic :)
This underlines the importance of context. A word may not be an acceptable translation sometimes, but may function well at other times. @Neha_Tyagi's example of "आप किस *सौदे (trade) में है" illustrates where it would be an acceptable replacement. However, the verb "to trade" may sometimes be appropriately translated as "सौदा (noun) करना" : "He traded his cart to buy a horse = घोड़ा खरीदने के लिये उसने गाड़ी का सौदा किया ।" It may also be appropriate in the derived word "सौदागर = tradesman". That may explain the option "trade" in the dictionary entry for "सौदा". ... I understand this 1-year old issue is dead :-)