VAT

Settings, Tax & invoicing holds your default rate and whether the prices you type include VAT.

Decide this before you enter your catalogue. Switching later does not recalculate your prices, so you would have to check them all.

A product can have its own rate when it differs from your default.

The margin scheme

Buying cards from private people means VAT on your margin instead of on the whole price. Mark that supplier as a private person; the lines from their batches are treated correctly on the invoice and in your numbers.