RV Trader is a good source for researching used RV. Rather than pore over dozens or even hundreds of irrelevant entries, you can set filters for MH class, diesel/gas, distance from location, etc.
Start simple and local with "
Class B" , "
Used" and "
100 miles" from his zip code. That should get him a few candidates at least. If nothing there suits him, then expand out to 250 miles. If there happen to be too many candidates, add in some more filters.
If cost is a key consideration, sort the resulting list from cheapest-to-most-expensive.
Also, eBay Motors works similarly to RV Trader in that you can set your filters to narrow down the choices. The good thing is that you're likely to find quite a few private sellers and you can look at their feedback to determine whether or not they're legitimate. The bad thing is that inventory of Class B motorhomes is hit-and-miss. You may have to be patient, waiting a few weeks for something suitable to come up for sale.
Another eBay plus is that the older RVs often come up for auction, rather than a set price (or "Best Offer"), so you can frequently find some real bargains.