Could you recommend a nice used USB DAC?

if you looking just for a USB input DAC, go for BorderPatrol DAC, its flying under radar, and very rarely you will find it for sale used anywhere online. Ask any senior / seasoned audiophiles, it is as good as $5-7k Dac for just $995 msrp. 

I used to have that Cd player years ago. The FMJ, full metal jacket. My sister bought it from me, I now use the Aloia for cd's.
Scott Nixon usb tube dac new is what I'd recommend.  I’ve been using Nixon tube dacs for years and they’re very affordable. I've never heard anything better. Legends...

http://www.scott-nixon.com/dac.htm

I just put in an order for the Ayre BQ-9 DSD.  Hopefully will get it soon. 

Finally I received the Ayre BQ-9 DSD. Here is my short impression and also compare bit with the Arcam CD23.First the Ayre is supposedly has its own USB power supply.  And also as important, the USB data line is optically isolated so it seems like a well careful design to shield itself from the noise of the PC side. Therefore the quality of the USB cable is probably not as important.  My impression is limited to red book cd only at 16bit/44khz since I do not have the software to play hi res audio.

First I really like the Arcam. It has both the details and at the same time sounding very musical - not an easy combination. Even though it came out around 2003, I still think it may be the best one box cdp at its price even now.

Most people have said cdp for the most part sounds slightly more musical vs. PC played through the USB DAC which is a bit more cleaner, analytical. I don't know whether it's a general truth, but I do find the Ayre follows the same line vs the Arcam. The Ayre bass is slight tauter, cleaner, may be slightly faster. The midrange is also slightly leaner or clearer (however you will). The very top end is a touch slight more open.
But on the other hand, I still prefer the Arcam. The Arcam is just slight more musical, more delicate and a touch sweeter. Through the Arcam you're just more there with the musicians. There are slightly more air and the instrument sounds a bit more natural. The Ayre on the other hand sounds cleaner and may be a touch more dynamic at the bass.
Other than that, both actually sounds somewhat similar. There is nother forward or harshness.

Hopefully I get to play some hi res on the Ayre soon.

First, my preamp is a Conrad Johnson 17LS and although it's a very good preamp, it's a bit laid back so it may not reveal all the details.  Hopefully I get a new preamp soon. 

After listening to the Ayre for a few weeks now I have come to the conclusion that it probably is a better unit vs. the Arcam CD23 at least judging objectively.  I mentioned that the Arcam has a slightly better present but after comparing that to the Ayre, it think the Arcam may have a bit of a upper bass lift so to make the sound just a touch warm.  But it may hurt its bass response which was not that apparent until I compare it to the Ayre.  The Arcam bass seems slightly congested and not quite as agile as the Ayre.  Also the Ayre also has more resolution on the upper mid and treble region.  You get more treble on the Ayre and there is a slightly better trailing on the treble.  The Ayre may also have a lower noise floor although I think some of it is masked by the 17LS.  Ultimate, I think the Ayre is more neutral and sounds slightly cleaner but just as smooth.
The interesting thing about this comparison is that even the Arcam is almost 17 years old, its sound can still be compared some of the newer units.  I think it may be let down by its power supply.  That maybe why Arcam had updated to CD23T version which has a better power supply and improved bass.  I also notice that the Arcam is a bit more sensitive to power cable vs. the Ayre.
Hopefully I'll get a new preamp which should reveal more about the difference between these two players.

just go for Wavelength USB Dacs
gordon ranking is the only man you can trust in usb dans

...and he morphs into gordon  skanking when you play roots through the Wavelength