Selling Uniques is a slow gold grind, if not mistaken the highest we can’t get from 1 Unique is 400,000 gold. While selling a stack of 99 shard of agony or the andariel doll lands around 8,000,000 gold.
So, stacking boss summoning materials is faster than selling itens (ticking to make it thrash/junk than selling in bulk). In the end 99 boss summoing will be sitting there anyway, unless you like the painstaking downtime farming Andariel, Duriel or Harbinger.