Learn Icelandic: Þetta er besta staðsetningin og besta söluverðið. - This is the best location and the best sale price.

Because it is using the weak declension. When you use superlative adjectives with definite nouns, you must use their weak forms. In the weak nominative singular, both feminine (staðsetningin) and neuter (söluverðið) adjectives end in -a. If it were a masculine noun, it would take an -i ending, like besti.

Why does the sentence start with the neuter word Þetta when the first thing it points to is feminine?

When you use a phrase like This is or That is to point something out and identify it, Icelandic always uses the fixed neuter pronoun Þetta, completely ignoring the gender of the actual noun being introduced.

Why do the nouns have the attached definite article (-in and -ið) when the adjective besta already acts as the best?

In English, the definite article sits before the adjective. In Icelandic, the noun almost always still needs its attached definite suffix even when a weak definite adjective comes before it. You are structurally saying best location-the (besta staðsetningin).

What makes up the compound word söluverðið?