That's great advice.
I always tell people who ask me how to test out products in the market to try and find a drop shipper for the products of interest. You may not always be able to, and there are other issues involved that you have to take into consideration when trying that.....pricing, service and so on, but it's an option worth looking at if you can!
Another way of looking at it is look for items that aren't drop shipped, let's face it, never going to find a niche product supplied by a drop shipper : )
Too many try and make the drop shipper their supplier, that's pretty much a losing game these days, but using them to test the market with very little outlay, then cutting them out of the loop when you find that market is just really good business sense.