If you offer it for sale, you should sell the monitor and computer separately.
Those KDS brand monitors aren't great but someone might be looking for just a CRT.
I'd suggest using your local craigslist so you don't have to bother with shipping.
This class of machine is fairly common at the moment, so the they won't sell for more than a few bucks. But at least it is beige! These Dells are a proprietary form factor (if you need to replace the motherboard, you have to get one from a similar model Dell), which detracts form the value. For this class of machine, cosmetic condition is extremely important, so if you want any hope of selling it, you will need some good high resolution pictures that show the machine is not scratched up or yellowed.
Is that internal modem an ISA 56K? If so, that would be worth mentioning. The ISA ones will work in DOS, unlike the PCI "Winmodems".