You don't need software that links directly to Ebay, Amazon to pull in the transactions.
However, it makes your life a lot easier if you do.
I've been using MTD now for the last couple of years and have rarely had a problem with it.
But I have a completely automated system now, Xero/LMB's.
From memory, there is some specific rules surrounding this, but it's not something I can remember offhand.
I'm pretty sure that it's covered on HMRC.
I know you have so don't say speak to an accountant, but though you may understand what your doing, they are the people that can guide you properly. And though it may cost you a few pound initially to get yourself properly setup, it will mean that you should get everything right first time. Something that I would prefer myself, rather than having to go back if a mistake is made.
https://www.gov.uk/guidance/use-making-tax-digital-for-income-tax/create-digital-records