I have received a few peaces from another personal tank in the past. All i did was ask the water perimeters and then placed the rock in a little 5 gallon tank for a few days to see if anything good/bad comes out of the rock. Unless they know the people selling the rock and know that they have a bad reputation i would say you would be ok. There are many good reasons not to except rock from another tank but i feel that all the reasons can be avoided. If you just want the rock and nothing else on the rock then you can bake the rock for awhile and kill off all the bad stuff on or in the rock, however like i said this will kill off everything that you are probably looking for or you wouldn't be buying live rock. My suggestion is to look into the people selling the rock and if everything is good with there reputation then buy the rock and place in a holding tank for about a week or two. That just my opinion though.
A few reasons why not to use rock from someone elses tank.
There's a risk that you could introduce some unwelcome hitchhikers, but this risk also exists with fresh LR.
As rock ages in a tank it loses some of the diversity it may have had when it was fresh, this can be overcome by also adding some some fresh LR in your tank as well. Small portions of LR should be swapped out for new every once in awhile anyway to maintain diversity.
If the aquarist didnt properly care for their tank the rock could also have been subject to phosphate buildup which could result in nuisance algae. If the rock is covered in nuisance algae you may want to pass on it, or you could follow a process called rock cooking which is a method of removeing detritus/nutrient buildup from within the rock.
On the plus side the rock is already cured, it should be available at a lower cost than new.
I probably wouldnt hesitate to purchase used LR from another aquarists tank. Make sure to properly quarantine it first as you should do with every addition to your tank.
__________________ ~David~
300+gal multitank, seagrass, mixed reef, clown anemone, refugium, and sump.
To me it looks semi cured. The first picture look ok. Looks like nice rock actually, However what is on the second top picture? Is the picture taken from the top and its just what i call creep foam?
Yes I think they are all taken from the top. The second photo is like a brown foam but if you click on the pic when its expanded I saw I think some bright red/orange spikes protruding from the rock ?
Roughly how much would you pay for all four boxes ?
Good new/fresh rock here goes from 12 -15 Euro a Kilo
The rock looks pretty good. Here in the States the average price per pound for cured LR is about 6 - 7 US Dollars per lb. I have no idea what the exchange rates might be. Prices are definately going to vary from place to place.
__________________ ~David~
300+gal multitank, seagrass, mixed reef, clown anemone, refugium, and sump.
12-15E/kg is about the same as our $6-7/lb. To give you some perspective, when I set up my tank I bought all my live rock for $3/lb from a couple who were expecting their first child and taking their tank down.
Well, my story is that when I got pregnant in '88 I tore my tanks down and was going to set them back up when the baby got a little older. He is 18 now and I just set a tank back up about a year ago.
The only question I would ask about LR from someone else's tank would be if they ever medicated the display tank. Especially if it was a FOWLR... make sure there was no copper involved.
Other than that I wouldn't see a difference between home owned and store bought LR.
Well, my story is that when I got pregnant in '88 I tore my tanks down and was going to set them back up when the baby got a little older. He is 18 now and I just set a tank back up about a year ago.
Hehehe i was born in '88 so im the same age as your son.
__________________ Im Like The Wicked Witch of The West Only Im A Guy And Have Evil Puffins Instead of Flying Monkeys
Ben
icon under their name to add to their reputation. 
so im the same age as your son.