There's likely nothing wrong with them other than the seller is attached to them and doesn't like the idea of them going for meat.
However, if you want to fill your freezer, you'll want animals you can either breed from or at least by small and fatten up before they go. Commercially, Zwartbles lambs tend to be sold straight away at weaning (or in our case a few months later), so you tend not to get them being sold as "store lambs" for fattening.