It's critical for ANYONE to be extremely familiar with their firearms. I tell my students to be more familiar with their firearms than their husbands, wives, boyfriends, girlfriends, etc. A firearm can kill instantly if used improperly, however the husbands, wives, boyfriends, girlfriends, etc. will nag you to death over a period of time.
All kidding aside, as the OP stated, be sure that you're able to properly operate the firearm BEFORE you purchase it. Most gun shops don't have a return policy on firearms. Once you've purchased it, it's YOUR GUN. You could attempt to resell it, but you will take a hit on the price. Regardless of how clean it is, it's still a "pre-owned" or "used" gun.
Another important point that was mentioned is that price shouldn't be the main deciding factor when purchasing a firearm. An extremely low priced firearm may be of very poor quality and could fail when it matters the most. I recommend that people purchasing a gun for SD buy a quality firearm made by a manufacturer with a proven track record. If all you can afford is a few hundred dollars, maybe a pre-owned quality gun would be right for you. If you want to purchase something new, then save your money for a couple of months until you have enough to purchase a quality firearm.
gf