Welcome to the wonderful world of plastic failures.

The problem will not get any better and possibly much worse. The piping have to be replaced and supported properly.

This is not an option as the new buyer has to be informed of the condition or the problem taken care of.

Hopefully it only a small section that needs replacement

It is far better to have a gun and never need it THEN need a gun and not have one

Check Hydro Physics. You will then really be confused. I just went through this, paying over $5,000 for replacement of a slightly bellied 4" pvc drain pipe running from the house to the septic tank, only now to seriously question whether I wasted over %5,000. In my case, our house was being sold, and the buyer insisted on having the system inpected and having a belly in a pipe corrected. Unfortunately I used the same plumber who said I had a belly problem to charge me over $5,000 to fix the belly problem. For me it was the same old, same old. If you ask a surgeon if you need surgery, you are going to get some surgery. I am guessing few drain pipes leading to septic tanks or public sewer lines are perfectly level, and practically all have some ups and downs or sags alias bellies. Be skeptical, get second opinions, require the plumber to qualtify in increments of .25 inches the sag, and negotiate the price.

Actually most sewer pipes do not have bellies or sags. And are graded at .25 per foot. When you run a sewer camera down the line there should be no liquid build up at all. If there is a sag than the solids will build up in that area and could eventually cause a clogged line.

It is not uncommon to find sewer and drain lines over the years do to settling and improperly installed being back pitched or dead level

If it is a small section and it is a combined system storm and sanitary there is usually not a problem as the storm water will help flush solids down stream. I personally do a lot of video inspections BUT I do not do the sewer piping replacement or repairs as it would seem to be a conflict of interest. I do tell accounts to look under sewer replacement companies and stay far, far away from franchises