Yeah I usually use a ball point pen. I’m thinking it might be the paper that these have been sketched on. Its quite thick (Muji sketch book) and so is not as thin as marker paper. I’m assuming and hoping I’ll get a better lineweight by sketching on marker paper.
I have also ordered some Berol Verithin’s (cheaper than Prisma’s) and so I hope to try and start sketching using pencil instead. I’ve been inspired by some of the lineweight that can be achieved by sketching with black pencil. I’ll hopefully post a few sketches next week when they arrive!
Thanks again all for the advice so far!