I believe there are a lot of threads on here re Bricksync, which may be the way to go - particularly if you want a presence on both marketplaces. For my part, I downloaded my inventory from BL as a XML, then imported into BO as a XML (and then closed my BL account <s>).
Thank u both, I got in contact with a guy who makes the program himself so that's the way to go but like Calibrick said, to get it all down I'll go the XML way.
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It may not please everybody, but its for me.
I use brickpacker.com for like 3 months now. Easy and stable, 1 month trial without fees, great support, Inventory copy, order sync etc
Myself is very happy now