How upselling and cross selling with remarketing ?
Here is are full documentation on Remarketing
How Remarketing Works
More info on Remarketing
In a nut shell, you utilize either an adwords script or the Google Analytics code to create a list of visitors to your website or certain pages on your website.
From that list ( based off of cookies) you can target those former visitors ( from your list) via the Google Display Network with Banner Ads that will display on any website that offer google ads to be displayed. You set the bid price for the auction space of those display banner slots.
There is also RLSA, which is Remarketing for search Queries that works basically the same way but utilized keywords and contextual ads just like the normal Search Ads program.
Google PDF on Best Practices for the RLSA
Please read though the documentation Links and If you have other questions just ask or Search for those answers that may already appear here on the forum.
Re: How upselling and cross selling with remarketing ?
You already have great advice from Eric. I'd just add here that up selling or cross selling requires targeting already converted users. To define this audience, you'll have to create rules (after placing the tag on your website), based on pages visited by a converted user. You already have this info in links Eric has provided, I'm just highlighting the importance of rules in this case.