D700 is/was a good shooter, and offers the full-frame advantage of a HUGE back catalog of lenses that are what they were designed to be, meaning, focal lengths and ranges FOR 24x36mm image area, like say the 28-105mm AF-D Nikkor zoom, or the 28-80 or 28-70 or 24-70mm zooms, or the low-cost 35-70 f/3.3~4.5 zoom, or the pro-grade 35-70mm f/2.8 AF-D zoom, or the 70-200 or 80-200 zooms, and the 20,24,28,35,50,85,105 lenses. Over 95%, as in more than 95 of 100 models for decades, were designed for 24x36mm capture area and shooting distances!
When you take a 70-200 or 80-200 and throw away the outside area of what the lens "sees", you have a lens that is no longer "right" for many places....the 24-70 is too long at 24mm to be a real wide-anbgle lens, but still not long enough at 70mm as a porteaiture telephoto, because the crop-body FORCES you to stand farther away from your subjects at every,single portrait distance. The 50mm is NOT a substitute for a high-quality 85mm lens. The 24mm is a nice WIDE, wide-angle on full-frame, but a bastardized mess on APS-C or DX as Nikon calls it.
Yeah, there are a few DX Nikkor lenses, but again, 95%-plus of the lenses in F-mount were designed to be shot on 24x36mm image area, at the distances we all know. The APS-C or DX cameras are fine, but the lenses are far fewer in number, and this is why a used D700 would make a lot of sense, because it could work so,so well with soooooo many lenses. A full-length two-person portrait with an 85mm focal length: 35 foot camera-to-subject distance on DX, but a mere 20 feet distance on a full-frame Nikon. Moving the camera back, from 20 feet to 35 feet, changes the way the depth of field is rendered, and means that many DX-camera shots have a more-recongnizable, and less-blurred background rendering than shots done from way closer camera-to-subject distances with a full-frame camera. Some people do not mind that DX camera look, but I do somewhat.
The D700 has a bigger, brighter, and BETTER viewfinder image than any of the D3000 or 5000 or 7000 series bodies; to me, the actual view through the viewfinder of the camera itself (not Live View) is a critical selling point,and makes me shoot better, or worse. For this reason, I dislike the squinty viewfinder pentamirror bodies, but some people shoot Live View, so that's an area where the newer consumer, mid-, and high-end Nikons have an advantage.
A D700 may be "old" to some people, but it was a high-grade camera. I prefer that type of D700 and the $3,000 camera body to a $349 or $599 camera body of the D3000 or D5000 series. The 7100 and 7200 were what I call the "$1,200 bodies"...nice, but not quite the same as the D700 was.