I do sell some of the armies that I buy. I once built up a Safavid Perrsian army in 15mm then saw some figures that were so much better than those that I had used that I sold that army bought and painted up a new one. I have also sold off armies from a lack of potential face to face opponents. I sold Warhammer armies becausee there was no one to play against then soon afterwards found a local club and built up new Warhammer armies to replace those I had sold.
I usually sell armies to free up space to store more armies. This is getting to be a problem as my armies increase in size. In the 70s armies tended to be in 25mm, the few 15mms available were simply not very good. As casting and moulding increased I moved to 15mm and then 6mm. A brief flirtation with 2mm did not last long. My eyesight is not as it was and these smaller models are now little more than black blobs. So I have been buying bigger figures. I recently gave FIW in 15mm a go. I bought and under coated Blue Moon 15mms then thought better. I sold these off at a loss and went for Redoubt replacements. Considerably more expensive and of course much bigger. All this has stretched the storage space and solutions available to me.
A popular local option are Really Useful Boxes. This is because there is a Hebden Bridge market stall that sells them at a pretty fair price every Thursday. The boxes are a decent height for 2 layers of 15mm but a little high for 1 layer of 28mms. They do stack really well if you keep to the same size of box and the colour coding can help identify what is where.
One use of the box is to fill it up with models wrapped in bubble wrap. This is pretty much the only solution for GW beasties such as Tyranids that are all angles and spikes. For more regularly shaped models the inserts that come with the boxes are OK but there will be a deal of sliding around without some sort of magnetic solution. Charlie Foxtrot models do cutouts in MDF for 1p or 2p pieces. These rely on the coins being ferrous and the use of strong magnets. I rate the solution fair for metal models. When built up with Basetex the coin bases do not fit that well. For lighter plastic models where there is less weight strain on the magnet bond I would expect better results.
A similar option is to magnetise the models to their movement trays and cram the trays into the box. This works as long as the box bottom is full and there is no chance of the box turning sideways.
The Really Useful Box is not essential. These Orks are in a box from a pair of boots. Be warned that cardboard will collapse if stacked high crushing their contents.
Out of the foam tray options one of the best deals are KR Multicase. The 4 tray 200 slot version is pretty good for Bolt Action. The size of my armies are largely governed by how much space is available in each box. Buying a new multicase is strongly linked to a major force expansion. The trays lie on their side when the basic box is carried. This may be OK for transporting a short distance but I can assert that carrying a army on its side through Manchester led to considerable miniature shuffling. This was with a metal Bolt Action army. I would expect more secure transport if a plastic army were involved. The tray slots are just about the right size for 28mms but there is some stress when fitting the bayonets in. The custom trays are a better fit for models but at some increase in price.
For smaller minis my money is on biscuit tins. The trick is to always use the same size tin so they stack well.
These are mostly 15mms, the keen eye may guess that some contain Dr Who minis. The tins are already ferrous so the magnetic paper goes on the model base. Many of the 15mm armies are DBM and have not moved out of their tins for years. I will probably have to chip them out but at least the paint jobs are sound and completely knock proof.