if you based in the UAE and your customers are also in the UAE and you cross threshold of AED 375,000, you may have to register. it is assumed that your products attract 5% VAT.

Irresepctive whether u sell online or offline, the obligation to register and obtain TRN arise once u reach the mandatory registration limit of sales value of AED 375,000. Therefore, untill ur sales turnover reach AED 375,000, you can sell online without TRN. Needless to mention, dont make mistake of collecting VAT and retaining the amount with u.