"Your total comes to 8.77." Stan the clerk said.
"And buying this will let me leave your store... without... issues?" Marty asked nervously...
"Almost, little human." Stan said unblinking. "Exiting, this establishment, with magic, such as the scroll you are purchasing, will keep you from having... immediate, unintended, consequences."
"So I can leave and just rip up this parchment, and I'll be fine..." Marty replied.
Stan's dull expression cycled through shock, worry, contemplation, and ultimately landed on bemusement before he cautiously said, "Humans and their short-sighted statements... YES. Though, for your sake, it would be strongly ill advised! You may certainly try, but, let me just explain, fate has granted you an opportunity to engage with magic on YOUR terms, if you squander this chance you will be targeted by magic in... other ways. You may be blissfully unaware as you become an elderly resident at a nursing home, or be painfully aware as you become the faithful family pet. You may become the a billionaire if your child wishes to be wealthy, or cease to exist if your aunt wishes she had a better relationship with her parents, and your dad or mom disappear from existence... Fate has chosen you, trying to walk away from this is... foolhardy at best." Stan laid the change from the 10 Marty had placed and pushed both the scroll and change slowly forward while not breaking eye contact. "Exchanges only, no refunds."
"So... one more time... I get a lock of hair or something from the person I want to exchange lives with... and recite the incantation and touch the scroll with their something... then..." Marty was suddenly more nervous about the implications of throwing the scroll away or back in the shop as he left.
"Then you will begin to become more like that person, and the more like that person you act, the more complete the transformation will be. They will become their version of you as you become their version of them...This scroll gives you 4 days. "
"What do you mean by version?" Marty asked paying more attention to the details this time...
"You will take on their defining traits... so if you swapped with your neighbor and they happen to be a construction worker, for example. You may become older, and burlier as you mimicked their habits and they will become some version of you. Eventually, you may take their job and place in society. You may or may not take their gambling habit but may end up loving beer like they do... it depends on your interaction with the spell, what you do will create your new life and what you fight against will increase your chances of... consequences."
"But I'll still be me right? After.." Marty trailed off..
"You will gain what makes them, Them. You will retain what makes you, you... Mostly... Probably. Everything is up for grabs. If you target a woman, you may become a woman, or remain a male. It all really depends on who you pick, why, and what makes that other person who they are. Life swapping is perhaps a bad phrase? Role swapping might be better. Who you are to others will be switched with someone else. If you are an only child, your father may not be a father after this. Someone else will take your role as student and child but may not take your place exactly..."
Marty grabbed the scroll and ran to the door wanting nothing more than to get out of that shop.