Well I looked through various guidelines relating to sololearn and technically no its not
I say if youre promoting a game, book, art piece, and similar that people have a CHOICE to buy its fine
And as mirielle said in your other question, some mods tweak the rules a bit to fit a certain situation so idk for sure.
A negative would be to put a link that a person gets money for every X clicks because those links usually lead to sites with malicious plugins, apps, executables, etc. and I believe someone from SL staff (Shahane) stated they did not like people using SL for their personal monetary gain.