So it preserves its victims while also digesting them?^ Given that the Sarlacc takes a thousand years to digest its prey, I can only assume it keeps that prey alive for that long. Otherwise the prey would die and rot. And Threepio did specifically say that the victim(s) would remain alive all that time.
Of course, this makes not the slightest bit of sense if you think about it for two seconds, unless Sarlaccs have some other form of sustenance besides (very, very slowly) digesting preserved victims.
Maybe they eat sand.