Mentioning Lego is absolutely OK, and you can sell used Lego as well and note that you are using genuine Lego bricks (resale laws simply allow that). Lego is really antsy about anything which might look like it is actually a Lego Group initiative though, and anything where Lego bricks are offered for sale in a modified state¹.

1: Never, ever, sell modified Lego bricks: https://www.brickfanatics.com/lego-wins-court-case-against-c...