Taking percentage based commission

I urgently need to know something before I fall into haraam dealings. Please tell me if my method of charging commission is permissible:

I have an online store where people can advertise their things on my website and I charge them on each successful transaction, so if something was sold for R100.00 I would charge R1.00 (1%)

According to the Hanafi mazhab we are allowed to take percentage based commissions from customers, I have collected numerous fataawaa on this Alhamdulillah. What I need to know is whether I can tell my customers this:

We charge 3% for any sales under R1 000, 2% for sales under R5 000 and %1 for sales under R10 000

Although no ambiguity remains and both I and the client knows how much will be charged at the end of the sale, I just need confirmation if is this permissible. See the case scenario below:

Sometimes the client will have something like "negotiable price" on his advert, so he would either sell it for R900 or R1 000 for example. Since I charge him 3% for sales under R1 000 and 2% for sales between R1 000 - R5 000, is this considered as ambiguity or is it permissible?

Another scenario:

Someone advertises an auction on my website. The price of the auction could be won at anything from R1 to R2 000 or more. Can I charge a fixed 3% on all auction listings so that no ambiguity remains as to what the seller will pay me? So that if the sale goes for R1 then I will get R0.03 and if it goes for R2 000 I will get R60

I am very cautious of falling into haraam activity and this is my income so I need it to be clean.