S5 1.15th $425 offer
SP31 5.5 th $1200 offer

5.5/1.15=4.48
4.78X425=$2031, even without extra PSU needed for S5
so, either you are overpaying for S5 or underpaying for SP31

I don't think that you would find many takers for Sp31 $1200 offer (unless BTC tanks)-just a comment.
Good luck with the buy