Why "greengrocer" kanjis are 八百屋 (やおや)?

I've seen the last one (屋) many times before meaning something along the lines of "establishment where you can buy things or services".

However, the rest of the word confuses me. Doesn't the word 八百 also mean the number 800 (はっぴゃく)?

Answer

Note: My answer is based on this entry for 八百屋 on Gogen-allguide

Originally it's called 青屋 (あおや) which is abbreviated from 青物屋.

There are two theories on how the あおや reading became the current reading やおや

1. Change due to the ease of pronunciation and for the purpose of differentiating from the indigo dyers which is also called 藍屋 (あいや) - short for 藍染め屋.



2. Change due to association with 八百{やお} which can mean "large number" corresponding to the fact that 青屋 sells a large number of items. This 八百{やお} comes from 八百万{やおよろず} which is a term meaning "uncountably large" used in Japanese religious context i.e. 八百万の神.
   
   
   
   • the reading of 八百 started out as やほ, then became やを, and finally settled at やお.
   
   
   
   

Regardless of how the change occurred, after the reading of 青屋 changed from あおや to やおや, the 青 was replaced by 八百 thus resulting in 八百屋.

p/s: As noted, 八百 and 八百万 denote 800 and 8,000,000 respectively. I believe their figurative meaning of "large number" derive from the religious connections though. For example, 七百万 doesn't have such figurative meaning.