Top end Raidhos have very poor resale. That is because noone pays retail on those. I've heard about people buying them 50% off from dealers. That is because dealers can buy them for 1/3 of retail - with a huge 66% off. They simply pass huge chunk of the cut to the enduser, who is then willing to part with them for 1/3 of retail.
The S7 got undercut by the S5 mk 2 heavily. Unless you have a huge room, there is little reason to get them over the S5 mk 2, at almost twice the price. The particular color doesn't help sales either. I haven't seen any single S5 mk 2 available for sale, but when they show up, I expect the price to be in $25k-$28k range which would mean very strong resale value.
Sonus faber latest top end models have lost their hi-end appeal. Plus, there is a sulprus of Sf on the market (due to bad company sales policy) and right now in Europe you can buy any model at 50% off retail, or even more (I've heard about 60% off with stuff coming from German distributor backdoor).