I've just bought a 1721 from Ebay with max memory(32F/128D), wic-1adsl & a wic 1enet
I bought it for 3 reasons - very cheap, keep my lab and home network connected to the internet (2 different subnets) & to practice setting up VPNs (to my 2620xm).
Fired it up and said image to big for available memory, and I was landed in ROMMON.
I when I finally found an image to boot the router & sh ver. It states I have 16MB flash & 32 MB Dram. Opened router up and memory DIMM slot is empty.
I looked on Cisco site and found that the 1721 should have a default 32MB flash and 64 MB DRAM.
Also when I save the config and then reload, it goes back to setup.
I think I've been had

Any advice other than hunt down the seller

P.S. Ebay description did say "100% working"