Each jurisdiction may have differing requirements.
If sale taxes or VAT are applicable, it might be up to the individual selling through BitMit to collect them, but BitMit probably has no responsibility to interfere. For sites like eBay with auctions, many of the sellers are individuals, these are personal sales (e.g.,, as a hobby, not as a business). To help ensure that larger sellers are booking revenues, compliance efforts occur through reporting by the payment networks (e.g., PayPal). With Bitcoin, visibility to a centralized payment system is not possible.
The method of payment wouldn't change whether or not taxes are due. The use of bitcoin can make it harder for a tax authority to know if all applicable taxes were being paid but it doesn't remove the tax obligation if one were to exist.
That's like asking please discuss legality and illegality issues of running a store that uses only accepts cash for purchases.