Three years? If you have no plans to use it, and it wouldn't reach for it as your first of second choice ride, then sell it. Try eBay or craigslist; you should find a fair and appreciative buyer. Right now might even be a good time to sell as the new Bromptons appear to be months delayed on delivery for special order and many shops are short on some models.

If you are publicly pondering selling, so what if someone makes an offer to buy? That is all part of sizing up your situation. An offer is what you want, not a sign of "greed" as another poster remarked (a sad testament to benighted cluelessness, if you think about it.)

What you have looks very nice and should get you a good price if you sell. Good luck with your decision. And remember, even if sometime down the road you decide you want another Brompton, there will be others to buy and modify, if you want.