Im going to assume your on your second play through...I wasnt aware you could sell the SDU's. hmm... I believe in the downloadable content there are a few claptraps you can save... not entirely sure though. The best advice I can give in search online and make sure you've found all the claptraps on both play throughs. Sorry man.
No worries dude, im pretty sure i have saved all of them wen't through 2 of the guides but i will look once more, i may be mistaken, I had done one as a co-op, not sure weather that makes a difference or not