The plural form (topwallpapers) would be better. However, it looks like you have a developed site there, but it's not being monetized? It would be better to monetize it and then sell based upon earnings.
I agree with ryan_uk though it is not developed it still have good potential. You would see lots of wallpaper sites with worst domains, actually the niche is very populated with lots of sites so you should be finding a interested buyer easily.