If your rear main seal leaks, the three things that can be done are to get it repaired, either by yourself of by a mechanic; use a leak sealer, which according to Auto-Rx may or may not work; or replace the vehicle. According to a mechanic on About, the best option is to replace the seal, using the sealer only if a temporary fix is needed.
The rear main seal, located at the rear of the engine, is meant to seal the oil from coming out of the rear end of the crankshaft. Due to its location, oil can quickly leak out, not only causing a low oil level that is damaging to the engine, but the oil that leaks out can fly into the rubber suspension components, quickly deteriorating them and causing even more costly repairs to be required. In addition to this, due to its location, typically the transmission and any related components have to be removed to get access. In extreme cases, the crankshaft itself may need to be removed, causing many mechanics to then recommend a partial rebuild of the engine in the process. According to Auto-Rx, if the seal is not torn or otherwise damaged, then leak sealers/reconditioners have the possibility of solving the leak. If not, then replacement is the only option.