Acts 20:30 (NKJV)
An exchange of one thing for another where you gain something, like when you purchase a product, is quite normal; you do this with the belief that you have gained something in the exchange. But what if you found out that what you thought was an exchange, or even something that seemed solely for your benefit, was just done for the gain of someone else? That would not be good because what was offered to you was done so with an ulterior motive, there were unseen strings attached.
While most things in the world operate on the exchange principle of giving one thing to receive another, in God's Kingdom it should be normal for us to do something that is a gain for someone else and we receive little, if any, personal benefit for ourselves. But, as I am sure you know, there are many people who are doing things for the benefit of no one but themselves. This is similar to what Paul told the leaders from the church in Ephesus when Paul said that people would say things just to gain disciples for themselves.
As Christians, and especially as ministers of the Gospel, our goal should be to teach and train others, even if that means we receive little or no direct benefit from those people for ourselves; we have to remember that God is our source. There should never be a situation where our goal in our preaching and working with others is just to build ourselves up, or to gain something from other people. We should follow the example of Jesus and do what we can to freely give the truth of God's Word to others, for the benefit of others, with no strings attached.