Pickup & tractor

Have you tried OLX?  They certainly have a load of tractors for sale, many/mostly used; and pickups, too (although a lot of vehicles are cross-postings from Standvirtual.

OLX is "the" place for buying & selling second hand - you'll need to use Portuguese, including when contacting sellers (as OLX "helpfully" warns users not to trust communication in other languages, presumably due to fraud).  Be prepared for some sellers never to reply, others to only want to communicate by phone, etc - and unless it says "negociável" it's best to assume they won't take an offer (although you can of course try!)

You may also find stuff on Facebook - both in Marketplace, but also many areas have a group called "vendas & trocas" or something of that kind - search in the groups for the name of your nearest main town, the common Portuguese term for your wider area (which may or may not be the same as the district/region)...  Areas more popular with immigrants may have English groups doing the same.  Some of these groups (PT or EN) have a load of events, clothes, etc and nothing of the category you're looking for; but others can be great.

@genki If you don't want to deal with private sellers on OLX, here's on OLX seller you might want to check ( https://www.olx.pt/d/anuncio/tractor-kubota-transporte-para-todo-pais-IDAJ6nM.html)

This resulted from a simple OLX search on "tractores", I have no experience with that seller, but he seems to have excellent ratings.

Regarding pick-ups, they can be quite expensive (from my point of view...): https://www.olx.pt/d/carros-motos-e-barcos/carros/q-pick-up/?search%5Border%5D=filter_float_price:asc

Those are great! I've never even seen a Datsun flatbed and that Citroen is cool.

Actually I was eventually gonna be looking to purchase a truck when we move as I will have horses and will need to get a trailer and be able to haul them.  The prices on those trucks are super cheap.  I live in Texas and currently own a Chevy 2500 HD and used is about $55,000. It is of course diesel.