Sample, sample, sample....check the entire claim. Check with BLM and the county recorder to be sure it's a valid, current claim. Check the surrounding claims and plot them out to verify any possible overfiles. BLM will tell you an overfile is a matter for civil courts and they won't be any help, even though the problem originated with them.
If the owner won't let you sample, run fast, run far.
Be aware that, unless it's a patented claim, all you are purchasing are the mineral rights. All other uses on public land are still permitted.
If you are satisfied, then it's a matter of negotian with seller. Lot of file and sell schemes out there. Look at how long the seller has held the claim....if only a year, that can be a red flag for me.
In the end you're better off doing research, prospecting and filing your own claim.
Look for closed claims, do some prospecting, if good, refile it. My own claim was filed, sold, and the owners let it lapse. Looked it over, filed on the half that was on the creek, and found out that everything upstream was available, and added 20 acres to the 20 I was filing on. Got 20 better acres in place of 20 useless, and for less than $400. Had I bought it when it was for sale, would have been around 3K.