Yes, this is good advice:

NEVER BUY AN OLD MODEL THAT HAS SPENT A LIFE TIME TRAVELLING FROM ONE SHELF TO ANOTHER EXHIBITION, NOT EVEN WHEN IT IS VERY CHEAP.

I bought such a model of the 1/48 monogram SU 25 recently, a real bargain. The box looked worn, dirty and old. I thought was the only reason for the cheap price (10€) as the content was still in its original, closed plastic bag.

However, now I know that the box showed that the content had suffered a hard and long shelf life resulting in deformed wings, fuselage halfs that are so twisted that when pushing one side together I have an opening of 3 centimeter on the other side, decals that I don't dare to use anymore...

I haven't given up on it yet but this one will require some serious work and a fair bit of luck to make something decent out of it.

Or, its not worth it to chase bargains like this!

Never happened on my old kits. I got a Tamiya Mobelwagen Quad type, the M3A2 Halftrack, M3 Grant, T-34/76 and the Panzer IV H made back in the 1970's. Fesh as new. Haven't tried it with AC's

well depends on the kit and the material. For instance even plastic figures or resins are not affected by time. Also metals are not at all, i have seen kits 10 years or more stocked in perfect condition.
Planes that have more flatwide surfaces and need tight fit probable have more problems with humidity sun exposion and all these

When you look at the box, and it looks like it has been through a war, then you can just about count on the contents being in the same shape.

Especially look closely to the sides to make sure it wasn't smashed, cause then you know you got trouble if it has. I got one, one time it had just a small crease in the side, and the store owner said it couldn't be damaged, cause look it is hardly creased.

Well when I got it home most of the parts were OK, but it was the real fragile thin pieces, that are the hardest to fix, that were all busted, it seemed as though the sprue was right next to that part of the box.

I had done told him everything was OK, cause I never looked at them tiny parts, I was only interested in the big main parts, and when they all checked out OK, I told the store owner they were OK.

It wasn't until months later and started to put it together and in cleaning, that I found them tiny pieces broke.
Then it was to late for recourse.

Now if I had seen them in the beginning, he would of traded me for another model just like it, but by the time I did find out, he had got rid of all the other models like that.

The problem comes from how the model were stored over the years, sitting in the overheated attic (Loft), or next the basement furnace, certainly doesn't prolong a stored models useful life. Even buying a brand new one that has been displayed in the shops sunny window will result in warped and distorted parts. The old saying of buyer beware, applies not only on E-Bay.

I'd bought a lot of old kits on Ebay and webshops, as many mates' advice on this topic, it depend on the condition of storage - only one Monogram 1/48 T-28 Trojan's decal manufactured in the early 80's was out of use but fortunately the seller sold it with a new aftermarket decal set.
Most of the sellers gave their advice on the kit with detail, condition of storage... I have no problem with them.