The CG63 was widely sold in the UK during the 1970s. This was a 7.62mm version with Parker-Hale sights called the CG63E, and we still see a few around usually sold pretty cheap alongside P'14, M1917, Mauser '98 and Enfield No.4 action equivalents.
It's a well made rifle, but is very much 1st generation TR kit. The barrel is light by later rifle standards, the bedding is basic to nil, the trigger remains standard Swedish service Mauser '96 with maybe some lightening of the pull. While a Carl Gustaf M1896 action was one of the best of the late 19th, early 20th Century repeater service rifle designs, it's weak compared to a modern single-shot action and the lock-time is very slow.
As soon as the South African Musgrave RSA appeared in the UK, the CG63 and similar became uncompetitive. The advent of the Swing / Paramount / RPA 'family' made it even more so, as did later actions and rifles built on the Barnard and similar.
I would imagine the 6.5X55mm versions as shown in the dealer's advert will be pleasant to shoot and pretty accurate if in good condition. It should be noted they're fitted with a Swedish Soderin target rearsight - a compact and nicely engineered device, but inferior to even the AJP and P-H sights of the 1970s - hence the CG63E having a P-H No.5 series rearsight fitted. I also doubt if the Soderin provides sufficient elevation adjustment to shoot above 600 yards.