If it continues into the summer and fall, as predicted, it will have a major effect.
The first phase is where larger gatherings like swap meets and rides drop off the calendar. The in-person auctions will drop as well. Online auctions will take place, but in some cases online is not yet a substitute for in-person (swap meets are a good example). If things are not contained, and the limits on gatherings keep dropping or if infections get worse, then one-off sales in-person will follow (craigslist, facebook, etc). Online sales will remain at that point, but if the carriers experience work stoppages, then online sales will drop too. We've gone from gatherings of 500 people, to 250, to 50, and now 10. That's a serious recommendation. The local Lowes has announced that if there is any exposure of employees there, the store will close for 2 weeks at minimum. If your local Fed Ex or UPS or USPS branch closes for several weeks, you're looking at serious problems.
So best case scenario, sales are flat and in-person meets and rides drop off. Worst case scenario - full-scale depression. We're in uncharted territory, at least within this lifetime because we have an economic downturn compounded by quarantines - not just a simple downturn.