I would say it only has to do with price point.

if a company started with only 1 photographer, or a select group, or even an elite group like some macro companies, and that 'elite group' companys old their images for under a price like $10.00 for a royalty free use, i would still call it microstock.

Inspired by the Wikipedia entry for microstock, I define microstock as having three required characteristics:

1. Images are sourced from the public. Macrostock is exclusive, only the cream of the crop get to participate. Microstock is inclusive, anyone can contribute. The 'microstock revolution' wouldn't have happened without this element. Throw in the crowdsourcing and Web2.0 buzzwords too.

2. Images are sold at micropayment prices. This means that the prices are so low they justify grouped payments. Macrostock prices (assumed to be $500 upwards) justify a single transaction, but buyers aren't going to put in their credit card details to buy a $1 image, and the transaction costs are too high for the agencies. This is why all the microstock agencies have 'credits' systems (or 'tokens', right Bryan?) with a minimum purchase at around $10.

3. Images are sourced via the Internet. This means posting in DVDs is not the only method of contribution. Most microstock agencies accept DVDs in addition to Internet uploads.

By 'required', I mean any agency that doesn't have all three of these conditions is not a microstock agency, in my opinion.