The sword in question appears in the season 6 episode, "Some Like it Not." At first glance, the seller identified it as an artillery foot soldier's sword from 1836. The item was originally purchased for a mere $75, but the seller estimates that he could get anywhere up to $800 for the extremely worn blade (which fortunately came with its original belt and scabbard).
Ever the military historian, Rick is incredibly interested in the blade. It is an interesting weapon from a time when America's army was still small, and as a result, the sword could be exceptionally rare. Usually swords of this era are curved cavalry blades given to officers and made for show, not standard-issue weapons made for a rank-and-file soldiers.
However, Rick is unsure if the sword is actually from 1836. The blade's tang is stamped with that number, but it isn't necessarily a year. Swords of this nature were distributed from the early to late 1800s, meaning that it could come from a post-Civil War time when such blades aren't all that rare. Fortunately, Rick calls in an expert who manages to confirm the sword is from 1836, not a later year.
The expert's opinion confirms that the seller's price estimate is about accurate. As always, however, Rick has to make a profit. After some quick and efficient haggling, Rick and the seller agree on a $500 price tag and both parties leave satisfied.