I am ecommerce operator of online scrap auction
We are providing service to conduct online auction all type of scrap for sellers/vendors through our portal/website, and get commission/service charge from seller and also collect amount/membership fees/participation fee from participants/purchasers/buyer 1-year membership for participation in auction. All seller and buyer registered in gst and have GSTIN No.  
Please give me 4 question answer given below

1. What is gst tax rate for me 18% or 28%
2. Our Recipients of service get full ITC (input tax credit)
3. Are we collecting same gst tax rate from sellers/vendors and buyers/participants?
4. we register our company in gst as Auction Service (auctioneer service provider). Are we required to register our company other than auction services in gst.

Thanks
Naresh
naresh.tatla@gmail.com