I haven't personally, but the chances of ever getting two loads of the same genre from the one supplier, exactly the same are extremely limited. It's just the nature of the business that loads will vary. Most suppliers work to ratios, so X amount of 1'st class, X amount of working, but missing parts, X amount of garbage that are only good for parts.
OK, that may not be the exact division of how a load is put together, but it's how most general loads in the surplus industry are done. If there wasn't a system in place to level out loads, then it would be all over the place as to what went out. When first getting into this market it can feel like a lucky dip as to what you end up with, and that's true to a certain level, but there is more of a system in place for general surplus loads than some people realize. With experience behind you, you can start to work out ratio splits if you are buying the same general stock from the same supplier. A lot of suppliers will even tell you the ratio split if you ask them as well, so I would always recommend getting that info upfront if at all possible so you are prepared as well as you can be.
Some suppliers will also put together tailored loads for you to meet your specific requirements. Just bare in mind that if you go in and ask for a load that is all 1st class, the price you pay will reflect your load.
The one thing about dealing in the surplus market is to remember it's a long term profit goal. Yes, you can make good money on your first load, but you can also lose on your first load, it's the whole lucky dip thing in action. In my opinion, to be really successful in the surplus market you have to stay in and push through. There are highs and lows, but as you gain the experience, you increase your chances of ultimate success.
In regards to the two suppliers mentioned, I'm not personally aware bstock, but genco have been around forever and are one of the big players in the surplus market.