I doubt if the remote site is looking at your MAC address.
I would be surprised if it were using your IP address.
It may be using a cookie to detect a repeat visitor.
With 99.9999999999999999999999999999999999999% confidence, I can tell you it'll be number three.
Number one is not technically possible, at all, ever. MAC addresses are OSI Layer 2. So we can rule that one out 100% guaranteed.
Number two is technically possible, but rather pointless to do, because of the effects of firewalls hiding multiple independent users (e.g. people at work surfing behind the corporate firewall).... hence most e-commerce websites will only use IP addresses as part of their fraud prevention measures at payment stage.... they are unlikely to use it during pricing (other than perhaps to automatically try to figure out which country you are in).
Hence we're left with number 3, cookies ( or their HTML5 equivalent, local or session storage).