> When making an offer on a MF property, is there any way to verify the rental income that the seller claims to make on the property?
Yes, you can ask for tax returns as part your due diligence documents in your offer. You can and probably should also get Tenant Estoppel statements from each of the tenants with them stating what they are paying and what they have paid in security deposit, etc.
> Is it appropriate to ask for tax returns?
Yes. You may get some push back on that.
> This is a small 8 unit property and the seller is apparently "old school" and collects rent in cash/money order and keeps hand written records.
OK.
> Come to think of it, he might not report all of the income on his tax return anyway.
Yes, that could be a challenge to verify it on tax returns, but the tenant estoppel statements should help some.
> I guess the only thing I can do is check out rental comps in the area and see if they are reasonable.
Yes, that's a good idea as well.
> I just don't want to get stuck with tenants who don't pay rent on time/leases that don't exist/rent amounts that are lower than seller claims, etc.
Yes, the tenant estoppel statements are statements made by the tenant to verify these things.
> I guess these are just the risks you take when investing in tenant occupied properties, though!
Yes.
> Anyone have any advice to offer?
Get copies of signed leases as well.
Hope that helps.