Well, at least the bores are chrome plated for added durability. Did you know that most consumer grade weed trimmer and blower 2 strokes run their pistons in a plain aluminum bore? They are emissions rated for 50 hours of operation whereas a professional grade trimmer with a plated bore are usually certified for 300 hours of operation. Not to say that you will only get 50 or 300 hours out of the engine, but they will remain emissions compliant for that period.
IMO, there are differences in the quality between different suppliers - but chef is correct that it is the seller that makes the difference. There isn't much profit in these things considering the small volume, and one siezed engine can wipe out the profit of several units sold. No profit = no motive to even bother with selling them. Most cut rate sellers on eBay state there are no warranties on these engines. They are cheaper because you, the consumer are assuming all risks. The better retailers charge more because they know that there wil be a certain percentage of defects and they have to cover their bottom line. No conspiracy, just plain business facts.