There are completists who collect M60 variants.
There are T161, M60, M60E1, M60E2, M60B (my favorite of the originals), C, D, M60E3, M60E4, MK43 Mod 0, MK43 Mod 1 (the MK43s are variants of the E4), and M60E6.
If anyone has any other variants, please feel free to add. These are all the variants I know.
As far as I know, the only new variants parts available are the E4 variants and the E6.
Other variants are very difficult to find parts for (especially the B stock, there are guys constantly on the lookout for the B stock)
So yes, there is a market for the E3 parts. If they're reasonably priced, you will find that they are easy to sell.
As long as we are on the topic of the M60, I hear alot of guys on YouTube saying that they can't 'Rambo' shoot the gun accurately. I find that to be untrue. The M60 is so controllable that it's very easy to walk shots into a target. Even if you use the sights, the sight tower on an M60 is so high that you're really estimating anyways.
It definitely is possible to 'Rambo' shoot an M60 reliably on target. And it's very fun.
The M60 is awesome.