https://wiki.openstreetmap.org/wiki/Tag:shop%3Dtrade has some suggestions, https://taginfo.openstreetmap.org/keys/trade#values has some in use subtags...

Some are different then what would go into shop= value, some match

And well, there is https://wiki.openstreetmap.org/wiki/Any_tags_you_like - so it is less about "invalid" and rather about "unnecessarily different from existing tagging for given thing"