A man standing outside the event Saturday bought the military weapon for $100 from another person there, according to Detective Mark Jamieson.
The single-use device is a launch tube assembly for a Stinger portable surface-to-air missile and already had been used. As a controlled military item, it is not available to civilians through any surplus or disposal program offered by the government, according to Jamieson.
Seattle police have contacted Army officials at Joint Base Lewis-McChord near Tacoma to deputy chief Nick Metz said Monday.
"Once it's brought on base and investigators have a chance to look at it, they'll see what they can determine," Army spokesman Joe Kubistek said Monday. "It's too early to give any information on it until we have hands-on access to see it and take a look at it."
Police witnessed the private exchange of the military launch tube near the gun buyback event, where gun buyers tempted those standing in long lines to turn in their weapons with cash.
"It was absolutely crazy what we saw out there," Seattle Mayor Mike McGinn said at a news conference Monday where officials announced they had collected a total of 716 weapons, including four confirmed as stolen.
Officers saw guns changing private hands without knowing whether the person buying the gun had the legal right to buy it, and those transactions are occurring all the time, McGinn said.
He added that the private sales of the missile launch tube and other weapons illustrate the need for comprehensive background checks as proposed by President Barack Obama, as well as other regulations at the state level.
While there were private gun buyers at the periphery of Saturday's event, Metz said a large majority of people chose to wait in line and get less money because they wanted to make sure they got the weapons off the streets.
"These are very dangerous weapons," Metz said. "They may not have looked very pretty, but (they're) definitely operable."
The firearms collected included 348 pistols, 364 rifles and three so-called street sweepers, or shotguns that include a high capacity magazine capable of holding twelve 12-gauge shotgun shells.
The program allowed people to anonymously turn in their weapons for a shopping gift card worth up to $200 -- $100 for each handgun, rifle or shotgun turned in, and $200 for each gun classified as an assault weapon under state law. Officials distributed about $70,000 in gift cards at Saturday's event.
McGinn said he wanted to plan another buyback event soon and urged more donations to the program.
Meanwhile, police said citizens who wanted to turn in guns could do so at any time outside a buyback program, though they wouldn't be compensated for it.