Online auction sites are useful when collecting an army. It's a fact, a lot of people use them to sell old armies and buy new ones (or new to them anyway), and we like these models to be in good condition, well assembled and not requiring a degree of repairs, stripping or other such time consuming activities.
But if you do happen to come across an army that isn't in the greatest condition but is at a lowr price, would you still go for it? This really depends upon how much time you are likely to invest upon a single army, but it can also sometimes be very possible to explain the bad condition of your army using paintjobs and back-stories, and I'm going to be quickly rattling through each of these here using my 2 favourite armies.
First onto my Orks. Out of my current 1,800pt army (excluding spare models, including an additional 10 Ork Nobz squad), the only things that are first hand are 8 Lootas, 10 Shoota boyz and 6 Nobz, out of an army that hits a model count of 74, which, admittedly is not huge for Orks but it's still a reasonably large size.
This was the army that I spent time repairing, as a lot of the second hand stuff was missing parts. Luckily, I was able to replace a lot of this with other pieces from my bitz boz (see below – I know it's big, I let people help themselves when they want certain bits, Monolith shown for size comparison, but do bear in mind I still have a whole Empire Knights kit in there and the annihilation barge part of the Catacomb Command Barge, as well as the Fantasy modular basing set).
But anyway, away from my bitz box, I am still in the process of making my Orks, as I wish to make some of their axes double-headed so that they can act as big choppas, and although this requires a lot of time, energy, glue and bitz, I feel that it can be worth it. It may not be the greatest looking conversion army in the world, in fact, it probably isn't that full on conversions, but at least it gives you a sense of pride in your army and the fact that it is unique and there is nothing else like it adds to this. Truly, no 2 armies are ever exactly the same, even if you have copied and pasted a net list the paintjob is never going to be an exact clone of someone else paintjob for the same list. But the conversions themselves, even if they're only small, make it even more special, and I would encourage anyone to do this.
If you can't be bothered with all of that modelling, then there is always as a second option. Create a reason for your poor condition second hand models to look the way that they do. I am of course talking about a back story, you can even create a paintjob to match this. I am currently in the process of doing this with my Necrons, as I bought them from my brother, and they were, unfortunately, poorly assembled. So, instead of going through the process of taking apart 40 Necron Warriors, 5 Destroyers, 2 Heavy Destroyers a Monolith, a Necron Lord and 2 Destroyer Lords, I set apart making a back story. Don't get me wrong, they weren't 'horror' bad, they were usable, but not ideal.
The back story that I made for this was that during the Necrons slumber their tomb world had gone into a state of ice age, and when the Imperium started to populate the planet, the heat that they brough with them due to the mass of bodies and the increase in industry, resulted in the ice in the North of the planet (where the Necrons slumbered) melting to some degree. This newly melted water made its way into the catacombs and caused significant damage to the Necrons circuits, ensuring that many of them were unable to properly regenerate from their slumber, resulting in some of their arms being slightly displaced or gauss that had been weakened. This therefore, explains why most of the Necrons are not assembled completely correctly.
In short, I would say, for a sheer hobby experience, it is always nice to find a second hand army, and show it some love.