As somebody who buys faaar too much on ebay:
Leave them in the blister unless you're going to paint them imminently yourself.
As to why... Well, these are reasons I will pay a little more for sealed blisters:
1) I can be fairly certain the figure is original and not a recast.
2) I can see that the figure is old, and not a re-release. The moulds of old figures wear out, and whilst some figures were sold by GW for a *long* time, the original releases were much crisper because the moulds were new back then, and made with original master models in good condition.
3) Old models were much softer than the newer pewter ones, and if they have been rolling around in a box with other figures for twenty years, the detail loses crispness. For organic fantasy figures, this might not matter much. But, for very small figures like Epic, or for figures where crisp, sharp edges are required (sci-fi or armoured models perhaps), the original packaging will have helped ensure that they've survived in good condition.
4) Certain models are notorious for having bits break off them, and it can be nearly impossible to tell if it's been repaired or not from an ebay pic. Odds of getting an unbroken model are higher if in original packaging.
5) Some models came with parts that were originally separate, or in a different material. Subsequent official models may have been all in one material/part.
Good example are Epic 40k Hydra flame tanks; the models (like most Epic Armageddon tanks that were re-cast in fewer parts than the original Epic 40k ones) had their tracks cast on to the hull... Backwards. And there's just about nothing you can do to fix the model without ruining it!
6) I can be fairly certain the model(s) is(are) complete. Some models come in a lot of parts, or come with a variety of options which you only use a selection of on the final model. It's therefore hard to get a "complete" models of this type if you are buying one that's already assembled on ebay.
So there you go; six good practical reasons to leave the models in blister, and why people like me feel that original sealed packaging is worth a couple of extra quid.
Of course, that's also without other considerations like the feel of opening a blister of old lead figures, or the nostalgia of poring over the old model pictures on the blister packaging...