Must an inground swimming pool have a main drain?

Yes it needs a main drain and a skimmer (one cleans from the bottom the other from the top) to do best job of staying clean.  With a LOT of extra work (basically vacuming the bottom with a suction sweeper) you could do what the main drain does most of (whereas you otherwise might vacum once a week or every two, depending on a number of things).

Do you know WHY and WHAT it is plugged with?

If just leaves and normal stuff, it should NOT be a bid deal to bet that out of there and unplug it.   if that's all, makes me wonder why seller has not done it.

Get a pool maintenance company to inspect it and give you an estimate on fixing it and then decide what to do.  As long as the pipe (probably PVC pipe) that runs from the main/bottom drain to the pool filter is not broken, its just not going to be that big a deal to 'un plug it" or normal debris.    Now it it was filled with concret that would be different.

Suggestion:   ask to cut the PVC pipe where the main line goes to the filter (it can be reconnected with a double female PVC connector that cost about $2 and a bit of PCV glue for $3).  After cutting it, put a hose with high pressure into the line back to the pool and see if it blows the stuff back out of the main drain and into the pool to open it up.   You might have to use duck tape to seal the hose in the PVC to get full pressure into the pvc line.  If you can't get water to blow through the plug up and out into the pool ... then I would have some concern about what is stopping it up and I would have a pool maintenance person come look and check it out.

Legally, if you are working under your INSPECTION PERIOD, I would write an addendum to the contract that says "Seller will have a reputable pool installer company give a firm price quote to fix the problem for less than $x and that buyer and seller will split that cost to get it fixed and operable PRIOR to closing and unless done and fixed, closing shall not take place."  or something similar, as you think works for you and as you can negotiate.    

Worse case scenaro, as I see it (but have a pool professional confirm this) is they would have to dig down on the outside of the pool where the pool main drain is (a deep hole, main is always in the deep end) and dig UNDER the pool to get it cleared or replaced and then run any new pvc pipe needed (maybe, not likely) and then fill the dirt back.   This could require emptying the pool (perhaps) and that would cost the amount of that water to refill it, plus the repair cost.  My guess is that no way could that go up to $1,000 ... but i am not in that business so you must check and be sure.

Note:  I am assuming this is an inground pool and that there is no reason that you cant dig down by the side of it.  If it has pool deck other than wood, there would be additional work and cost to break into the deck to dig down to the main on bottom and the cost then to patch where the deck was broken up, a hole big enough for a man to get down in it and to then dig UNDER the pool to get access to the pool main ... again this is unlikely worst scenario.

Good luck,

Rob

The main drain should be covered with a pool drain cover that is designed to prevent entrapment. Is this what you mean by it being plugged?