HE parents who are in need of tutors usually start by asking around within the home ed community, because they want a tutor who is aware of the issues which affect home educated kids and has experience with home education. Without the involvement of schools and teachers, it's all the more useful to have access to a knowledgeable tutor.
For example, few exam centres accept private candidates for GCSE English Language, and those are likely to charge a good deal more, so most people prefer one of the IGCSE options instead. One hears stories of families who wrongly assumed their tutors would know all about the process of entering for exams, and many months are wasted during which the tutor prepares the children for an exam they won't be able to sit!
They would expect their tutor to have some familiarity with the different exam specs and experience with tutoring for whichever one their child is going to do. There's a fairly high rate of special needs among HE children, so it's also good to have worked with kids who have autism/anxiety/dyslexia etc. HE children can sit exams at any age, so it's helpful to be able to assess their readiness and advise on whether they might be better off waiting a year. It's a bonus to have good contacts at local exam centres so as to have some warning of which ones might be about to start or stop taking private candidates for the relevant boards, which changes from year to year.
All of that explains why people are cautious about employing a tutor who is new to home education, with the exception of obscure subjects where there's less choice of tutors. Those are just a few thoughts based on what I've overheard; home ed parents spend a LOT of time talking about exams! 😆 I haven't employed a tutor myself because my child was self-taught for English Language IGCSE, so I don't know all the details.