As with most of these calculations, you are right to say without thinking about lots of other variables. I think that there are a few that need to weigh in a little bit.
The biggest one is access to either of these types of stores to buy the products. For me, IKEA is a two hour drive each way. I could get to a Target or Walmart in 15-20 minutes, so it would probably be better to get self-assembly furniture there instead and save the longer trip.
Likewise, buying used can be even closer to home, especially if you are buying second hand from others. However, in terms of driving time, you likely will buy from a diverse group of people to get all the furniture you need, which could mean lots of individual trips, versus one trip to get the self-assembly furniture.
So in terms of initial purchase time and driving time, this could learn more towards self-assembly in my book.
The second step is then the actual moving. Again there are two options, sell or ship. For shorter moves, I would guess that it is better to just move the furniture versus again driving all over town to get "new" furniture (really just different if you are buying used :) ).
In terms of selling your furniture, I would guess that there is a decent disparity between the price that you can get for a piece of self-assembly furniture v. a used piece of quality furniture. In my experience, self-assembly furniture is at least half price what you would pay in store if buying used. Standard furniture you can seomtimes sell for as much or more than you bought it used. (This is probably more the disparity between new and used than a direct relation to quality. Also it is far easier to look up the price of an IKEA piece of furniture than other used furniture, so the "new" price is more known.)
Not that price is the ultimate decider in terms of sustainability, but I find that it has a definite influence on the ability to accept a green practice. When someone knows that they will be hard pressed to recover the cost of a product and deem it fairly cheap to begin with, it is more likely to be thrown away than sold or shipped.
Ultimately, my primary consideration (having moved 3 times in the past year), is the ability to pack around the furniture. I had some furniture from IKEA that we disassembled and moved around. I also had some used furniture that I packed. I also had some furniture of both that was either seriously worn or cumbersome to move, so we donated that to a local thrift store.
If you plan to be moving often, I would want to limit the number of moving trucks and I think that this can be done with either type of furniture as long as you are willing to pack in around the furniture (which may depend on the moving company more than anything). We did a U-Haul, so we were able to pack it full and move it well.