Error 73 fires when the listing price is below eBay's minimum (typically $0.99 USD or 1.00 EUR/GBP). The listing is rejected until the price is raised.
Most often this is a transformation bug: a feed accidentally exports cost-of-goods or a $0 placeholder, a sale price is calculated as a percentage of an empty list price, or rounding produces a sub-penny value. eBay won't let any listing go live below the per-site floor.