After pulling up times on National Rail for a R/T journey from London to Cambridge, it asks you to choose a website to purchase your tickets, which directs you away from the National Rail site. Does anyone have a preference on these sites? Some of them include First Capital Connect, The Trainline, Silverline, Virgin Trains, and more.

Another issue is when I try to book on any of these sites, it forces you to register and wants to look-up your address before purchasing, which isn't working since I'm in the U.S. I can't seem to get to a spot where I can purchase the tickets.

Can anyone help or suggest an alternate website to book tickets on?

I've purchased from the trainline with success. I had the tickets shipped to my first hotel.

You can use any of them to book any train ticket in the UK but my preference is to book with the company that runs the particular service as you may get discounts are available.

The trainline charges for CC bookings BTW

In all cases you have to register which means having to provide a UK postcode - use your hotel or some random one.

You might then find that at the payment stage your CC is rejected because the address doesn't match. In that case amend your address to match that of the CC.

However I'm not sure that booking in advance will save you any money - many of the shorter trips on UK don't have really cheap fares booking in advance.

And finally, DON'T use raileurope, I've never known them to offer good deals on UK travel especially once they add seat reservation fees

Trains from London to Cambridge are frequent commuter trains with no seat reservations, so no reason to buy tickets in advance. You just buy a ticket at the station and hop on a train.
If there are any special offers for tickets, they will be found on the train operators' web sites. The National Rail schedule indicates the operator. For the two routes to Cambridge, these are www.firstcapitalconnect.co.uk and www.onerailway.com.

Thx everyone-- you were all most helpful!