One of the most common questions (of that sort), here on AB, is:
"What are the pros and cons of trading as a limited company instead of as a sole trader?"
Another possible question might be:
"I'll be selling secondhand goods which I've purchased from members of the general public. (i.e. not from VAT-registered traders). How do I calculate the VAT which I'll have to pay to HMRC?"
(Remember that you'll need to check that the secondhand goods don't include vehicles, as the rules are different for motor traders).
Another possibility:
"I understand that I can submit vehicle expenses to HMRC either on the simple basis of 45p per mile for the first 10,000 miles (and then 25p per mile over 10,000 miles) or by recording all of my expenses for fuel, insurance, servicing, etc, individually. What are the pros and cons of the two systems?"
One more (which I've known HMRC staff to get wrong!):
"I'm the sole director of my own limited company. Do I take my pay from my profits and declare it on self-employed basis or do I have to get my company to pay me via PAYE as an employee of that company?"
. . . and yet another:
"I'm filing my online tax return and I see that I can use the "cash basis" for doing so. What are the pros and cons of doing it that way?"