Conflict of interest is a universal governance issue, it doesn’t just apply to strata, it is up to individual schemes to determine how they deal with it. If you are concerned about how these matters are handled, one approach is to have the EC adopt a conflict of interest policy that sets out how it deals with it. There are plenty of examples on the internet on which you can base a policy.
Personally, I do have an issue with an EC member having a direct interest in the provision of services to the scheme. While it can work, and may in some instances cost less, it can create problems. For example, if a lot owner provides building services and there are defects that may lead to litigation. I agree with Whale and PeterC that in many instances an EC member may be doing the right thing, as it benefits them as an owner, but it can be tricky.
Two of us on the EC are in professions that could be used by the OC, but we would never offer to supply any services, because we would be worried about liability.
Certainly an EC member who is quoting should not have access to other quotes, because they are may use that confidential information for their own personal benefit. Similarly they should not vote and should absent themselves when the vote is taken.