"When I was using Quickbooks self-employed I just categorised it as cost of goods sold. But here I find it harder to choose a category."

If you have Quick Turnover, and only Buy to immediately sell, then it still is appropriate to post to COGS. That just means "expensed" and not really On Hand over any length of time. The goal is to match the sales to the COGS date, or you really do have product sitting on the shelf over time and should be carrying it as inventory, even if you want to manage this as Generic or Periodic Inventory value.

You need to ask your own Accountant about these facts.