I've been successful as an engineering consultant for more than 10 years because my primary concern has been solving my clients' problems.  I try to give my clients the most effective solution for their money. If buying, or modifying, something off the shelf is faster or less expensive than having me design something custom to solve the problem, that is what we do. 
If connecting a client directly to someone in my network will most effectively solve the problem, that is what I do.  If I add value by facilitating the work, I will, or if the client wants "one stop shopping" I will be the interface for them; otherwise, I make the connection and get out of the way. 
Being primarily a one man band, I have to be careful that I don't over commit my time.  Some of the things I do to make sure I can meet all my clients' deadlines are:  1)  being realistic about how long things take to get done, 2)  keeping travel to a minimum by doing as much as possible by email, over the internet, or by phone and 3)  taking on only one project with significant travel requirements at a time.