No iPads are locked but:
Like earlier iPad models, the iPad 4th Gen and iPad mini models still are not locked to a carrier. However, LTE capabilities are different and each configuration is limited to carriers that support a particular LTE band.
Specifically, the A1459iPad 4th Gen (Wi-Fi/AT&T/GPS) and A1454iPad mini (Wi-Fi/AT&T/GPS) mostly intended for use in the US on AT&T and in Canada on Bell, Rogers and Telus -- support LTE bands 4 and 17 as well as GSM/EDGE (850, 900, 1800, 1900 MHz) and UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz).
Taken from:
http://www.everymac.com/systems/apple/ipad/ipad-faq/ipad-design-info-font-where- to-buy-unlocked.html
So if you're selling your iPad include the above info.