Question about buying a posh frock and resale

[absolutelyknackeredcow]

I have two properly black tie events to go too this year - one is abroad- and originally I was thinking about hiring but given timeframes of how long I will be away I think this is going to be pricey
Has anyone got experience of buying a reasonably high end dress - eg vampire wife - and reselling ?
If so what kind of drop on resale did you have to take?
Thanks xxx