Do we need to cancel promotion created on marketplace before creating in SelluSeller?

No, you don't need to. If the promotion created in OMS (SelluSeller) overlaps with Marketplace campaigns, then Marketplace campaign will take precedence over OMS (SelluSeller) promotion.

For e.g. if you have created a percentage dicount of 10% in SelluSeller for the product and now there is also a Marketplace campaign of 20%, then in this case, Marketplace campaign of 20% will take precedence and the product gets purchased with dicounted price of 20%.

We hope the details above were helpful for you. Please feel free to share with us if require further assistance and will be happy to take it up.