Can you post some pics of the stove? Most stoves are sold because they have a bad build up within them (plugged with ash). Have you cleaned the unit entirely? From intake to exhaust ?? Do you know the exact exhaust path that the stove has? Some stoves have very tricky exhaust passages and need some creative tricks used to properly clean. A clean stove is a happy stove.
With that said, pellet build up is an airflow issue. Which can mean a couple different things? Either the intake is blocked (doubt this one)' the exhaust path is plugged up (new venting, so doubt this also), the stove is plugged with ash (used stove makes this a very good candidate), or there are leaky gaskets (also very likely do to being used).
Perform the dollar bill test on the door and ash pan gaskets (if it has an ash pan).
Take a dollar bill, open door to stove (Not running obviously) place dollar bill in area where gasket will close on it, close door as you normally would, then try and remove the dollar bill (or piece of paper), if it pulls out easily, the door needs adjustment, or if no adjustment, then replace gasket.
After that, clean the stove, then clean it again. Find out how the air exits the stove. This is vital for an owner to understand. The air comes in through the pot, then from there, it varies by stove make and model, but takes a path, or series of paths, to get to the combustion blower.
Also, is the burn pot sitting in the liner properly? Any air that is leaking by the pot, reduces the combustion air for pellets. A leaky gasket, burn pot not seated correctly, and a dirty stove, will build up pellets very quickly.
Do a search for Leafblower trick here. If you have an electric leaf blower with vacuum option, it's a great tool to have, to help you give the stove a little extra "zip".