DId you get that paperwork from the IRS about purchasing? over $600.00

When determining "profit", is the sales amount in nominal dollars or inflation adjusted dollars? If you buy something for $1000 then sell it after inflation is declared at 10% for $1050, have you made a "profit"? The dollars you were paid are actually worth $945 (adjusted for inflation) compared to the pre inflation $1000.

So who pays the taxes, buyer or seller?

Seller; that's the person with (potential) income.  If I sell something here and don't have a record of what I paid for it, I'm unlikely to accept PayPal.

recapping what's already been posted:  The IRS is after income and not imposing a "Sales Tax."  So keep note of what you paid for it (like stock transactions) and profit (income).  If you sold it for $1000 but paid $1200; that's a $200 loss, no income tax.

As for Sales Tax that may apply depending upon State/Local, but I doubt locals can track online sales yet.  In both cases the seller pays Income Tax or Sales Tax if any.

(Not an advisor, just by take) 😉