You can use "suffer", but it is not that common to use it in the present tense as you have. If you did, you'd more likely include "from" and say "I suffer from buyer's remorse". In the present tense, you usually say instead, "I've got buyer's remorse" or "I have buyer's remorse". The past tense is a different story and 'suffer' is more common than what it was in the present tense. ie "I suffered from buyer's remorse" is very common.

I'm experiencing buyer's remorse. (most common) I'm suffering from buyer's remorse. (ok, but 'suffer' is too strong if not delivered humorously. 'sale' can replace 'activity'

Yes. I suffer from buyer's remorse.