Mark up on material by trades as an owner builder

It depends.....
They should be charging you some $ for their time and resources in sourcing the product. After all, it does cost something to get the products. That price may be included in their hourly rate, or a separate charged item, or some other way.
Best to simply ask them whether it is best for you to supply the product or for them to supply. If they are honest they will give a prompt and honest answer.
Sometimes a supplier will only sell to you at a retail price whereas a tradie will get a discounted price.
Sometimes the tradie will refuse to warrant their work if you supply the product. That may or may not be reasonable.
If a tradie tries to hide their costings from you then I recommend you find another tradie.
Good luck. You'll need it as an owner builder since, in Australia at least, every one you deal with will try and rip you off as much as they can.

I built cost plus with a family friend so saw all the prices as well the discounts they get through suppliers.
In most cases the builders margin was less than the discount the builder got - so it was always better to let the builder supply materials.

Always get multiple quotes, no matter how reasonable/cheap you think it is. I remember getting a few quotes for a pool fence, roughly around the same price, thinking the third one will be around that mark so why bother - I went and got a third quote, was much cheaper and great workmanship. So it's always worth seeking various tradespersons, and even negotiating with supplies (their suppliers might be heaps more expensive, so if you can source it yourself, could save heaps).