I think, as a general rule, that the components for sale here and on USAM reflect what was popular during the period when the components were new. Botique audio companies don't publish their sales figures but ARC is a popular brand and they have sold a lot of gear. I remember when this preamp was introduced and it got absolutely gaga reviews. I'm sure that a lot of people rushed out and bought one and now some of those owners have moved on to the next hot thing and they are selling their ARC gear.
I peruse the AGon and USAM listings once in a while and I've seen times when there seems to be a lot of units of a particular component for sale at the same time. With the amount of gear that cycles through these sites it is not unusual to see a disporportionate number of a particular model for sale. In a couple weeks it's something else.