Deepcycle batteries are the way to go for sure, but car batteries will work awhile as stated also.
When I first bought my place in MO. I had a trailer house, no power, no well, nothing but the trailer and land! We only came here a few times a year for a couple weeks. I used a 300-400watt inverter on a car battery and it worked well for a long time. I had 2 batteries, one charged to drive the van on and the other to use for the inverter. The van was set up to charge both at once while driving to town and back. Also I rigged an alternator to a riding mower for a gennie when we needed to charge a dead battery and did not need to go anywhere. Used the system many times, same batteries for a few weeks at a time like that, then for a good while when I moved here till I got powergrid installed. Gee those were the days eh
That was years ago before internet was even wide spread.
Anyway I am sure it was not good for those batteries but I never killed them either, they lived a long life in normal Auto use plus the inverter use like that.
If buying batteries for long term use get deepcycle, if just testing stuff short term limited use and they are cheap or free then auto batteries will work too, but not really recomended.
Shallow cycling does help them also as stated, but in the use I used them in above I ran them till the inverter shut down (often) and I did not kill them, though they probably had a shorter long term life than they could have had if used properly.
Also they most likely will not have as many amp hours as a deepcycle which means they will go dead faster.
If buying new, Wall-mart sells a 12V 115amphr deepcycle for about $55-$60, I would buy that over a $35 auto type anytime! Free or recycled I would use anything that works. Look for golf cart dealers and you might be able to get good trojan T105s for around $10 each, but would need 2 of those since they are 6V.