If you are attempting to sell your car it is worth making sure it doesnt leak or there is nothing missing that should be there
. If you have a buyer checking out your car and they notice that there is fresh oil underneath the car then you will have a lot more trouble attempting to sell your car. If the buyer looks carefully and notices that the car is suffering from some damage to the under carriage then they are probably going to have even more reason to not even make an offer. The under carriage can get damaged by things such as flying stones. If the car has problems with the suspension then it may bottom out going over speed bumps or larger rocks.
Another thing to be aware of when you want to sell your car is the condition of the exhaust. If the exhaust emits blue smoke or if the smoke burns your eyes it can be indicative of engine trouble. If the exhaust is corroding and has holes then the buyer may be required to purchase a new exhaust in order to get the car registered (so they comply with emission laws and general safety requirements). Normally certain parts of the exhaust can be pulled out and replaced and it wont cost a lot. However, if the whole exhaust needs to be replaced it can cost a great deal and may not be worth the buyers money. If the exhaust is in good condition it does a lot to show the buyer how good the cars condition is when attempting to sell your car.
Look out for loose parts that may be dangling from the under carriage. A weld may have broken or in the case of the exhaust hanging off, a rubber ring that secures the exhaust the vehicle may have snapped. There is no easy fix for a lot of these things. In fact the under carriage itself is one of the hardest parts of the car to keep an eye on. This is why a lot of mechanics put your cars up on a hoist when inspecting it for road worthiness. The average person cant afford a hoist so it may be worth it to get a mechanic to check it out underneath.