Thank you SO much for pointing me to this post which I'd completely overlooked. For the benefit of other readers of this discussion, it's at https://forum.duolingo.com/comment/32162103 and is... completely essential. Lingot given, and many thanks too!
You have bought them and
You bought them are technically distinct in English - past perfect vs simple past I absolutely agree, and you're right that i had missed that point so it was definitely worth making... but in practice, in this phrase the difference in English meaning is so subtle, and the past perfect form (at least in the uk use, in conversation) is falling out of use, so Im honestly still not sure :-)
I defer to your grammar skills but i was taught the best test for past perfect is to add 'just' - eg 'you had JUST bought them in the market... (eg) so i didn't tell you they were on special offer at the shop', and yes there's a distinction, but it feels pretty slim.
Anyway, thank you, much appreciated, and you definitely taught me something here!