Can I sell my off-plan unit before handover?

Usually yes, by assignment (transferring the SPA to a new buyer). Most developers require you to have paid a minimum share of the price first, commonly 30-40% (verify against your SPA), then issue a resale NOC for a fee of roughly AED 1,000-5,250 + VAT. The transfer is registered via Oqood at the DLD. Some developers restrict resale until a set stage, so check your contract.