Post the link of the tank and well tell you if its a good deal or not. Like someone said some people who sell their tanks.. Think they are going to get all their money back.. No one is going to buy a 3-4 or 10 year old 30-40-50g tank for $200.. And like someone said usually these people will give you all the equipment.. The lady who I bought my 55g from for $120 gave me the lights, pump, filter, air bubble pump and nets and so forth..
My 44g Pentagon for $100 he gave me everything as well including like 4 power heads filter, the quad light, and heaters and nets and air pumps... Of course this time unlike the 55g all the crap he gave me didn't work, except the heaters and light but that's ok
As for cleaning the tank, obviously drain it take the rocks out and put them in a bucket.. or throw them away if you want new rocks. To clean the tank when you get home what I do is set it in the front yard get the water hose out and bottle of bleach and I fill the tank like 1/3 of the way and pour half the container of bleach in there.. then I get a sponge with the soft side and scrubby side and I scrub the walls using the bleach solution in the tank., scrub the bottom.. scrub the top plastic parts and the outside glass.. then I let it sit for like a couple minutes.. then I tip the tank over gently and drain the water and with the other hand I spray the entire inside of the tank to get all the bleach water out.. Then spray the rest of the tank to get the bleach water off.. Then I bring it inside the house and fill it up.
Btw don't SCRUB the seals.. just the glass
If your going to reuse the old rocks.. I suggest you actually put those rocks in the tank when your cleaning the tank with the bleach solution and give it a good swirl.. Badically you are disinfecting everything.. make sure you clean the rocks good of the bleach though.. Then when you fill the tank up I use 2 capfuls of prime, and then put a handful of food in the tank and let it cycle for a few weeks before I add any fish making sure the ammonia and nitrites are gone first.
As for the used filter.. toss the old filter pads out and buy new ones.. and just wash it in the kitchen sink with hot water and sponge.. NO SOAP
This is how I cleaned my 55g and my 44g pentagon, I just got this pentagon with 2 heaters and a QUAD lamp for $100 like I said, I reused the old rocks too as I liked them. I washed everything just as I described above.. even the gravel.