What is the difference between 交易就告成立. and 交易即告成立.?
The only difference between 交易就告成立 and 交易即告成立 is that the former is more used in spoken language and the latter in written language. 即 in 交易即告成立 is a very formal word and functions as an adverb, meaning "right away/at once/immediately/thereupon". When 即 is used in this sense, usually it can be replaced with 就. There is no difference between 即 and 就 in their meanings or grammatical roles in this case. Another example:


  It took the police only several days to break the case. 该案仅几天即告破/该案仅几天就告破(了).


When used in some idioms, for example, 一触即发, 一拍即合, 稍纵即逝, 即 can't be replaced with 就, though 即 still means "right away/at once/immediately/thereupon" in these words.

As for CIS即视觉识别系统, 即 is a conjunction here. In the construction A即B, 即 roughly means "namely/is also called/equals". B explains or rephrases A. It's not the same with 交易即告成立. 就 can't mean "namely/is also called/equals", so it can't be used in place of 即 in this case.

Your native friend may simply base his/her answer on intuition. If you need an exhaustive explanation, you'd better refer to some Chinese grammar books. @user6065 gave a good reference book 汉语800虚词用法词典.

Edit:

I beg to differ with @wpt's claim that the use of 就 in 交易就告成立 is a misuse influenced by dictionary definition.

Here is the definition of 即 in Contemporary Chinese Dictionary, 6th Edition, page 605.



⑥&lt;书>[副] 就;便：一触~发 | 招之~来 | 闻过~改。

&lt;书> means this meaning of 即 is usually used in written language. [副] means 即 is used as an adverb. 

Here, the dictionary used 就/便 directly as the explanation of 即. That's just because 即, 就, 便 all have the meaning of "right away/at once/immediately/something happens soon after another". Except for some idioms and Chengyu(成语), wherever you use 即 in this sense, you can use 就 or 便 in place of 即. The three words are interchangeable grammatically and semantically. As I just said, the only difference is that 即 is more formal than 就 and 便.

There is a paper on the evolution of 即, 就, 便 from verb to function word (adverb and conjunction). cf. “即”、“便”、“就”虚化过程中的错项移植. With regard to when 就 was first used as adverb, the scholars haven't reached an agreement. The author of this paper believes it's no later than Song dynasty (960-1279).

@wpt claimed that 即告 is a set phrase. My opinion is that 即告 is just a commonly used phrase, not a set phrase. Less Google search results doesn't mean 就告 is a misuse. Greater occurrence of 即告 in formal documentatuions doesn't mean it can't be replaced with 就告 gramatically and semantically.