The domain aftermarket is a genuine asset class with brokers, marketplaces, escrow services, appraisers, and a body of sales data that’s scattered across half a dozen sources and agrees with itself about half the time.
The plural in this name is deliberate and useful. There isn’t one aftermarket; there are several, operating on different platforms with different pricing conventions and different buyer pools. A site named for the plural is positioned as an overview rather than as one more venue.
It fits a marketplace aggregator, a sales data and comparables service, an industry publication, a broker directory, or an education resource for people entering the business. Aggregation is the most useful: seeing inventory and completed sales across venues in one place is something the industry still lacks.
Two words, plural, no hyphen, .com, and it’s exactly how people in the business describe the business.
Offers considered.