It looks like wordpress with some modifications and a custom theme.
Or at least it can be made using wordpress and a few plugins - in this case, one domain is a page/post and the only thing it renders its a contact form for that domain.
In my opinion it would be more logical to auto-populate a single site-wide contact form with metadata associated to each link/domain. You can do this using the
Contact Form 7 wordpress plugin associated with
Contact Form 7 Dynamic Text Extension.
I've been developing a poor-man's wordpress "showroom" website that basically helps eventual interested visitors in searching the whole portfolio - it's a work in progress and so far it's very easy to build: generic theme adaptation, only a few plugins, alterations kept to a minimum - little hassle.
Its assumptions are that domains are aggregated by similarities (unique characteristics, patterns, TLDs, industry, etc...) in one post, and each domain links to flippa's respective sales page - for reputation purposes and eventual offers/escrow.
You search the portfolio by tags (keywords, TLDs, Registrar, BINs, etc...) and use the contact form to start inquiry or buy (not yet working).
You can see it here:
CardboardDomains.com.
