With the meaning of "last XX", "dernier" is always placed after the noun:
- lundi dernier, la semaine dernière, le mois dernier, l'an dernier.
With the meaning of "(on) the last XX", "dernier" is placed before the noun:
- le dernier lundi, la dernière semaine, le dernier mois, la dernière année.
"We have sold" in present perfect suggests that the time span is unimportant or that the action still has effect at the present time.
"Last Thursday" gives a precise time for the action which is now finished.
The past simple is used when an action is one-time, past and complete: "We sold it last Thursday".
The present perfect would be used with no end-date for the action, "we have (already) sold it".