I am familiar with this situation.
First, inventors may as you sign a non-disclosure agreement. This is not fishy, in fact, I have recommended folks to do so before showing me the parts.
STLs are digital files, and can find their way into the public domain, and with 3d printers can be easily duplicated. If the customer is looking to do an invention, he is smart to ask you to do an NDA. This is common for prototyping businesses.
Now, I have also had folks that have asked me to do business outside of 3dhubs. I respectfully decline, and here is why:
1. I am not interested in NOT paying the sales fee. That is, 3dhubs gets the sales commission.
2. Buyer protection is important to the customer. If the customer is unhappy, 3dhubs will jump in and make a decision, as a third party, as to whether I have failed the customer, of if the customer has failed me. This has never happened to me, but it is a good reason to do business through 3dhubs.
3. 3dhubs collects the fee up front, key in me getting paid. I also do work outside of 3dhubs, obviously. But if 3dhubs brings the customer, I will not work with them outside of the process. In fact, the only one time that I did not get paid for a job, was a direct customer who artfully manipulated me, and I did not get paid. 3dhubs has this protection built in.
4. I do good work. Thus, the review is important to me!
5. I do meet with customers. However, I have had two situations in three years where I simply did not feel comfortable. If that is the case, DO NOT DO IT. In both cases, I met at the local restaurant near my house.
6. 3dhubs brings me good solid business, and the fee they get helps us all. They are great at support, investigation, payment, content, and advertising. They also developed the software we all use, and maintain that software.