Smart contract development

about creating smart contract for custom token and nft marketplace
do we need to deal with auditing? If so pls. give us more info how this auditing work?

It is up to you. Some users will want contracts audited for their own peace of mind especially when it deals with significant amounts of money.

Audits are done by third party groups or companies for a fee where they go over your contracts and make up reports with their findings, any suggestions, etc.