On Friday night whilst feeding I noticed one of the spiders (1cm sling) I recently obtained was hanging curled up from the little piece of cork bark in the deli cup.

Carefully prodded it and it was extremely lethargic. After trying to move it settled back into the curled up position.
Put it in ICU where it curled up even more and ceased movement altogether. I left it alone and checked in every 8-12 hours or so.

Saturday afternoon it was writhing all over. The movement was fluid, but everything from the fangs to the spinnerets were in constant incoherent motion.

Sunday morning it was sitting high, like when spiders feed. On careful prodding it tried to move with typical DKS type jerky movements the collapsed back into a curled up ball.

I left it all of Sunday. Opened the ICU this morning and it was sitting like, well, a spider. Tried to run up the side of the ICU but with DKS type movements, so I left it and will check in tomorrow morning.

The other two sacmates I bought are doing well under exactly the same conditions.

I am at odds whether to continue though.

In an eggsac of 200+ spiders I accept and even expect there to be a small percentage that would be deficient in some way. These slings would never make it in the wild, possibly to the betterment of the species as a whole.

What I am doing now, is taking exactly such an individual and artificially providing it with the conditions to prolong its life. If this weak one is to survive and breed, would I not be doing the species in the hobby a disservice by possibly promoting this defect to continue within its progeny? I can't help but think that, whilst it would be great to save this sling and watch it grow to a beautiful adult, it might not be the best in the long run.

Lots of 'possibles' and 'perhapses' I know.

The seller has graciously offered to replace the sick spider, so money or investment loss is no contest.

Should I continue, or allow the spider to die?

I would try keep it alive. If it makes it to the next molt and recovers, cool. If it has the same symptoms after the molt, put it in the freezer. If it is a weakling or has a genetic disorder, chances are it won't reach adulthood anyway.

Edit: Thumbs up to the seller by the way...

I would agree, give it a chance, sounds like it got better from when you found, so maybe just needs some time.
Like Scarab said, it might pull through or it will die, the only way you know if you try.

If it makes it to an adult T, should you not know better and not breed it?
I would try everythign to save it - the whole point of captive bread animals is to give it a better chance to survive.

i agree with you Karin,,, my girls mom brought home a puppy Jacky the one day we had one look at her and saw she had two extra toes on the hind legs and nine nipples,but we kept her(and omw she is the cutest little thing ever)but we would never breed with her! so i say too,help the little guy out, his yours to take care of! u never know it might be a stunning T! with no probs later,,,

Who says DKS is hereditary?