Can Apple Store assist with unlocking my iPhone?

Return it if you can. The phone is useless to you. If you can't return it, then you have unfortunately learned a valuable lesson. A legitimate seller will give you your money back.

Apple does not get involved in any third party sales. I could create a receipt myself for a device and it means absolutely nothing.

Besides, if they unlock it for me, that would not be helping the thief as they had already given it to me

What? That would absolutely be helping the thief as they were still able to profit off a stolen device, and would continue to do so as long as they could find someone who would be willing to buy it. You are out of luck.

PeaceNchang wrote:

I completely understand that, but if I were to show proof of payment and explain my situation could they not understand were I'm coming from? Besides, if they unlock it for me, that would not be helping the thief as they had already given it to me

No, showing you paid for the phone is not good enough. The only documentation Apple will accept is the Original Purchase Receipt from when the phone was originally purchased, which shows the iPhone Serial Number on the receipt. Without that, a trip to your Apple Store will be a total waste of your time as Apple will NOT unlock the phone for you.

You've been had here. The phone is useless to you. If you can get your money back, try to return the phone for refund. I'm sure whoever you bought it from won't give you your money back. That leaves using the phone as a coaster, or dropping it by any electronics store, such as an Apple Store who will recycle the phone responsibly for you.

You should read this --> The All Too Common SAD Reality of Buying … - Apple Community

The seller had a phone they knew couldn't be used. Most likely stolen. Now they made it someone else's problem by selling the phone.

Apple will not for any reason clear the activation lock. For all they know, they could be helping a thief get into a phone that doesn't belong to them.

I'm afraid you've been taken.

If what you said is true, either the seller is the dumbest person in the world, or you got scammed. Either way, you have a brick whose anti-theft mechanisms will keep it bricked.