品揃えの多さについつい買いこんでしまい、...
I ended up unintentionally buying ??? complete set...
I'm struggling to understand the part in bold. I know that 多い is 'many', and that you can convert adjectives to nouns by changing the last い to さ. So I get 多さ = 'manyness' (obviously not a real word) maybe 'multitude' or 'multiple'??
Now I have a 'multiple of complete sets' with a に on the end. I can't make any further sense of it.
I also wondered if when you add a さ the thing you make is a na-adjective rather than a noun so I could make an adverb from 多さに but that didn't help either.
I also notice there's no object marker を.