Mossberg anyone?
Seriously though, upon my first outing with my new Mossberg 590 I experienced a double feed and a few misfeeds. Needless to say I was quite upset and ready to dismiss the 590 as crapola. But, when I tore it down, I discovered that the mag tube spring was all bent to h*ll. Plus, I noticed the elevator needed a bit of polishing. So, I replaced the spring (per various recommendations given to me) and polished the elevator a bit and things have improved so far.
Keep in mind, this gun you have is man made and thus, prone to malfunction. Through trial and error (as frustrating as this can be) you will discover the cause for your various issues. After that, its entirely up to you as to whether or not you consider it worth fixing. Personally, in addition to my previously mentioned "fixes," I have spent several hundred dollars upgrading my standard 590 to 590A1 status. Sure, I lost a bit of ground, financially speaking, but felt that the weapon was inherently worth the trouble vs trying to sell it.
You need to keep in mind that the 870 Express, along with guns such as the Mossberg 500/590, are "budget" weapons vs the likes of the Remington 870 Police models or 590 A1 models. Hence, QC will probably be somewhat lacking and unfortunately, make the buyer responsible for dealing with any missing and/or lack of details. So, dont write off the gun just yet.
I doubt if it needed a cleaning as it was "just out of the box" new and assembled by my dealer
This means nothing as far as Im concerned. Being "assembled by your dealer" could mean anything. If he just attached the barrel, this could hardly be construed as a noteworthy assembly. Plus, most new weapons contain all sorts of junk, both externally and internally. I would not make any assumptions, but rather would break down the weapon (as far as you feel comfortable with) and thoroughly clean/lube it. After that, see what you end up with and go from there. Just be sure that, when cleaning the mag tube or any part contained within, that you leave it clean and dry...no lube necessarily needed. With all other parts, get them as clean as you can but do not over apply the lube, particularly when dealing with the trigger assembly mechanism.