We've searched the internet and found the largest collection of On30 Scale trains for sale is on eBay. Below are the listings for 'boxcar'.

BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 605
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 605
$68.07
Time Remaining: 1d 10h 45m
Buy It Now for only: $68.07
Sn3 Colorado  Southern CS Box Car 2 Assembled Unpainted Couplers  Trucks
Sn3 Colorado Southern CS Box Car 2 Assembled Unpainted Couplers Trucks
$45.00
Time Remaining: 3d 23h 34m
Sn3 Colorado  Southern CS Box Car 1 Assembled Unpainted Couplers  Trucks
Sn3 Colorado Southern CS Box Car 1 Assembled Unpainted Couplers Trucks
$40.00 (3 Bids)
Time Remaining: 3d 23h 45m
Sn3 Partially Built Box Cars Scratch built out of styrene no trucks or coupler
Sn3 Partially Built Box Cars Scratch built out of styrene no trucks or coupler
$30.00
Time Remaining: 4d 1h 46m
ON3 ON30 SILVER CITY MODELS DRGW 1872 12 4 WHEEL BOX CAR KIT
ON3 ON30 SILVER CITY MODELS DRGW 1872 12 4 WHEEL BOX CAR KIT
$36.00
Time Remaining: 4d 14h 16m
Buy It Now for only: $36.00
Spectrum Bachmann 27635 ON30 Freight VENTILATED BOX CAR C  D NEW in Box
Spectrum Bachmann 27635 ON30 Freight VENTILATED BOX CAR C D NEW in Box
$44.95
Time Remaining: 4d 16h 5m
Buy It Now for only: $44.95
Spectrum Bachmann 27618 ON30 Freight VENTILATED BOX CAR DRGW NEW in Box
Spectrum Bachmann 27618 ON30 Freight VENTILATED BOX CAR DRGW NEW in Box
$44.95
Time Remaining: 4d 16h 20m
Buy It Now for only: $44.95
Spectrum Bachmann 27656 ON30 Freight VENTILATED BOX CAR BREAKWATER NEW in Box
Spectrum Bachmann 27656 ON30 Freight VENTILATED BOX CAR BREAKWATER NEW in Box
$44.95
Time Remaining: 4d 16h 56m
Buy It Now for only: $44.95
A Plan For VIctory Box Car Hawthorne Village Civil War Confederate Express NEW
A Plan For VIctory Box Car Hawthorne Village Civil War Confederate Express NEW
$49.99
Time Remaining: 10d 21h 43m
Buy It Now for only: $49.99
Stonewall Jackson Box Car Hawthorne Village Civil War Confederate Express NEW
Stonewall Jackson Box Car Hawthorne Village Civil War Confederate Express NEW
$49.99
Time Remaining: 10d 21h 44m
Buy It Now for only: $49.99
Bachmann Spectrum 27636 Ventilated BoxCar Lawndale On30 Scale
Bachmann Spectrum 27636 Ventilated BoxCar Lawndale On30 Scale
$32.98
Time Remaining: 12d 48m
Buy It Now for only: $32.98
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
$68.07
Time Remaining: 12d 15h 50m
Buy It Now for only: $68.07
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
$37.00
Time Remaining: 16d 17h 21m
Buy It Now for only: $37.00
Bachmann On30 Scale Train 18 Freight Cars Box Car 2 per box 26501
Bachmann On30 Scale Train 18 Freight Cars Box Car 2 per box 26501
$91.82
Time Remaining: 17d 20h 24m
Buy It Now for only: $91.82
Bachmann On30 Scale Train 18 Box Car W Murphy Roof Gray 2 box 26553
Bachmann On30 Scale Train 18 Box Car W Murphy Roof Gray 2 box 26553
$91.82
Time Remaining: 17d 20h 24m
Buy It Now for only: $91.82
Bachmann On30 Scale Train Box Car South Pacific Coast Sunset Logo 27002
Bachmann On30 Scale Train Box Car South Pacific Coast Sunset Logo 27002
$45.91
Time Remaining: 17d 20h 26m
Buy It Now for only: $45.91
Bachmann On30 Scale Train Box Car Mineral Red Data Only 27098
Bachmann On30 Scale Train Box Car Mineral Red Data Only 27098
$45.91
Time Remaining: 17d 20h 26m
Buy It Now for only: $45.91
Bachmann On30 Scale Train Box Car Oxide Red Data Only 27097
Bachmann On30 Scale Train Box Car Oxide Red Data Only 27097
$45.91
Time Remaining: 17d 20h 26m
Buy It Now for only: $45.91
Bachmann On30 Scale Train Box Car White Pass  Yukon 27001
Bachmann On30 Scale Train Box Car White Pass Yukon 27001
$45.91
Time Remaining: 17d 20h 26m
Buy It Now for only: $45.91
Bachmann HOn30 Scale Narrow GaugeTrain Box Van Blue 77202 Thomas
Bachmann HOn30 Scale Narrow GaugeTrain Box Van Blue 77202 Thomas
$48.17
Time Remaining: 17d 20h 30m
Buy It Now for only: $48.17
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
$68.07
Time Remaining: 21d 11h 51m
Buy It Now for only: $68.07

Other On30 Scale train items recently purchased through this site:

Sn3 Partially Built Box Cars Scratch built out of styrene no trucks or coupler
Sn3 Colorado Southern CS Box Car 1 Assembled Unpainted Couplers Trucks
Sn3 Colorado Southern CS Box Car 2 Assembled Unpainted Couplers Trucks
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
ON3 ON30 SILVER CITY MODELS DRGW 1872 12 4 WHEEL BOX CAR KIT
Spectrum Bachmann 27635 ON30 Freight VENTILATED BOX CAR C D NEW in Box
Bachmann Spectrum 27636 Ventilated BoxCar Lawndale On30 Scale
Spectrum Bachmann 27618 ON30 Freight VENTILATED BOX CAR DRGW NEW in Box
Spectrum Bachmann 27656 ON30 Freight VENTILATED BOX CAR BREAKWATER NEW in Box
A Plan For VIctory Box Car Hawthorne Village Civil War Confederate Express NEW
Bachmann On30 Scale Train 18 Freight Cars Box Car 2 per box 26501
Bachmann On30 Scale Train 18 Box Car W Murphy Roof Gray 2 box 26553
Bachmann On30 Scale Train Box Car Oxide Red Data Only 27097
Stonewall Jackson Box Car Hawthorne Village Civil War Confederate Express NEW
Bachmann On30 Scale Train Box Car South Pacific Coast Sunset Logo 27002
Bachmann On30 Scale Train Box Car Mineral Red Data Only 27098
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 3145 n
Bachmann On30 Scale Train Box Car White Pass Yukon 27001
HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
Bachmann HOn30 Scale Narrow GaugeTrain Box Van Blue 77202 Thomas
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 605