Are these Side Shells from 1948?  

Nope, looks more like the...
Nope, looks more like the 1960-70's to me, or maybe even later...

the early ones
I have seen were on X bases so I would also say no.

But either way they look...
But either way they look great and a nice price!

Ummm
We're still fuzzy on whether or not shells were even available at that time....

More info
OK, seller has told me that 3 of the chairs are from Vitra and the white one he believes is older HM with no label. He also has a 2001 Noguchi rocking stool (the shorter one) from Vitra. He has made me a final offer of 900 CHF for all 5 pieces (approx 800 USD). Do you think this is still a good deal? The three chairs, for sure, are plastic, not fiberglass.

I said no.
I turned him down. I thought that was a bit much for used plastic Vitra. Should I kill myself now?

Nope, you should live.
Not a good deal.

No
The armshells themselves weren't in production until 1950, so it is highly doubtful that the side shells were from 48.

Actually,
You know what I think? I think the seller looked at DWR's webpage...which, for some reason, lists the shell chairs as being designed in 1948. I KNEW I had seen that date applied to the shells somewhere... oy. morons.

That's exactly what...
That's exactly what happened! I poo on vitra plastic. No worries. You did the right thing,

I trusted my
instincts and what I have learned from y'all over the past two years and I just KNEW that that was too much money for not enough "ooomph", if you know what I mean. So, I live to barter another day.

Plus!!!
I forgot to tell you! He was like, all uppity and sh*t in his e-mail, saying "well, if you knew what I was selling" and "if you knew how significant these were" and trying to be all that and a cup of coffee. And I was like, dude!, your chairs are plastic!
Ah, me.

What a nerd. Just give him a...
What a nerd. Just give him a link to this site and tell him a card carrying member. He'll know you know more than he initially thought.
BTW - Plastic Vitra Eames chairs are not very significant at all. If they were vintage however ....

On second thought, Riki
Did the seller tell you that the three are indeed plastic? I'm asking because it looks like all the chairs in the pics have shock mounts. The current Vitra plastic version does not use shock mounts.
If you have clearer / closer photos, it should be pretty easy to make out the material.