Just be up front, and a contractor may not want to work that way at all. Sometimes getting deals on stuff is going to cost more, in my world a plumber needs to order everything.The issue is when something shows up wrong, they will not want to be responsible for it.
Communication and a log book are keys to a good informed job. Also when you throw the DIY in there, don't expect them to come running when you have an issue.
Just tellin you like it is, but don't be discouraged, do your homework and you can work with a good contractor. That is the most important, build that relation and they will help you out, but do it wrong....they won't be there for long.
Bottom line....DIY takes longer and sometimes is not worth it on a bigger project. Do the things you know how to do and do not hold anyone up.
Let us know when you talk to some contractors.