What types of customers have you worked with?
Apart from community projects, the most common job I take on is fixing peoeple's web presence by building a new site that integrates with social media.
What advice would you give a customer looking to hire a provider in your area of work?
Demand that a vendor explain everything you need to know in a way you can understand. To not do so means either they lack communications skills or are being dishonest, expecting you to just "trust them". And whatever you do, do NOT buy based primarily on price because technology services can rarely be compared fairly to each other.
What questions should customers think through before talking to professionals about their project?
The one thing few people realize is that good web sites are rarely made by geeks or artists. Geeks make sites that are technically correct by whatever technical benchmark they follow, but may not be usable by everyday people. Artists can make amazing visuals, but often do not consider loading times, screen resolutions, or properly structuring the site to be viewable by everyone, including the visually impaired or others using text-only or smaller devices.
What people need most is someone who understands how real life business and communication processes translate to the web -- and give back measurable returns.