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.