Releases can boot from RAM disk. The not yet released 0.4.15/16 branches can boot natively from USB on some older hardware (with EHCI or older USB controllers). I would not recommend "buying" ReactOS regardless, unless the install media is officially distributed (which AFAIK is not currently).
I asked the seller and they said the USB flash drive has the iso files on it; both the bootable and the live versions. I have a feeling it was not bootable but just storage for the ReactOS files.