Which design of alloys are they?
I may be wrong but I thought the offset of all genuine VW 7.5J x 18” alloys for the mk7 / mk7.5 Golf was ET51. An offset of ET48 suggests they might either be replicas in the style of genuine VW alloys or if they are genuine VW, then they might be from a VW other than a mk7 Golf. If they are replicas, make sure you don’t pay over the odds for them - i.e. don’t pay the price that you’d pay for genuine VW alloys. Replicas might also have a different centre bore size to VW OEM alloys (VW centre bore is 57.1); if so, then you may find VW centre caps won’t fit.
One way to check if they’re genuine or not would be to ask the seller for pictures of the reverse side of the wheels. If they’re genuine VW, then they’ll have tell-tale markings cast into the reverse side of the spokes, such as VW part number, VW logo, the manufacturers name (probably one of Ronal, BBS or Borbet), the wheel spec details - diameter, width (the J size), and offset and usually a ‘JWL’ safety certification mark. There should also be a ‘KBA’ certification number on the face of the wheel on the edge / rim - usually near the valve stem hole (KBA certification means the alloys meet all the safety requirements established by the German Traffic Laws and is a mandatory certification for alloys sold in Germany - which genuine VW alloys for the Golf will be).
As for the difference in offset; IMHO I very much doubt you’d notice.
