• Not trusting themselves (Beginning verb-ing modifier modifying the subject)
  • to choose wisely (modifies “trusting”)
    • among the wide array of investment opportunities on the market, (modifies “choose”)
• stockbrokers are helping many people (Main clause. Subject – Stockbrokers; Verb – are helping)
  • who turn to them to buy stocks (Relative clause modifying “people”)
    • that could be easily bought directly. (relative clause modifying “stocks”)

As is, the sentence says that stockbrokers don’t trust themselves to choose wisely among the investment opportunities in the market and that they are helping many people who come to them to invest!

Clearly, there seems to be a problem with the sentence. If we did intend to say that stock brokers don’t trust themselves even though they help others, then we should have presented this information using contrast words, not a verb-ing modifier providing additional information about the clause. It seems that the initial verb-ing modifier needs to modify “people”, who don’t trust themselves to choose wisely and thus turn to stockbrokers for help.

The use of conditional “could” (“could” can either serve as the past tense or conditional form of “can”. Here, clearly, we are not talking about the past) is correct here since the part using “could” is presenting a hypothetical situation (In reality, the people are not buying directly).

Also, note that “not” (acting as an adverb) before “trusting” is modifying “trusting” (an adjective). We can have adverbs modify verb-ing modifiers. For example: “completely trusting”, “more trusting than…”, “Rarely trusting” etc

The interesting thing about this sentence is that it has only a meaning error, no grammatical error.