A tough choice I think. On the one hand, I think your domain is worth more than that. e.g. FindMe in online sold for $7500 a couple of months ago, and your name strikes me as somewhat comparable. Back in July 2016 ShareIt and PayIt both sold in online for a bit over $300. On the other hand, sales of .online are not easy to come by, and $100 is not that out of range often paid for them.
If it was mine, I would counter offer something enough to keep them interested, maybe a high $$$. Good luck no matter what you decide to do.