Hi Lena and welcome to the SaleHoo community,
I would expect perhaps not so common, but you make a very valid point in regards to checking product quality.
Wholesaler's in general will usually send you a sample product first before you outlay a large volume of cash on stock sight unseen and tested, but you will have to still pay for it in most cases of course . So if you transfer that rule to Drop Shippers, it essentially involves you just ordering the product to test it for yourself before offering it for sale.
Could you imagine how much suppliers would lose if they happily just gave away free product to everyone that asked, that's a nightmare no supplier wants to willingly be a part of.
So I wouldn't be holding my breath waiting for a Drop Shipper to offer free sample products any time soon :)