I agree, probably nothing you need to worry about. I asked a similar question a couple months ago: Rust On A New Van
. Peter at SMB also said this was normal.
If possible, have the owner email you some pictures. Also ask if you can take the van to an independent mechanic for a pre-sale inspection. This will give you a peace of mind as far as the actual van engine, brakes, and other components are. Also, make sure to do a carfax report.