I think opened and/or seriously damaged boxes take 5 - 10% off what you would get for a MISB set.  Add in the time it takes to verify completeness after opening the set, and that's why I don't do it.

​It's not necessarily about completeness but about making sure there's sealed LEGO bags inside, so you have at least seen it once - and maybe took a picture (instead of counting every piece), so you can be sure you don't sell something totally different

This idea has been talked about for years, but like then is still very much a non issue. With experience you can see when a box appears to be tampered with - if so, don't buy it. If you purchase it online, and it looks questionable - verify it.

If it makes you feel any better, please feel free to open all of your sets. I have also seen amazingly resealed plastic bags over the years - so you should also open all of those. The pieces could also be fake chinese ones (substituting the rare ones), so perhaps building the set might be the only way to know for sure that they are real LEGO (as the quality when built will be obvious - at least it will for now).

This idea has been talked about for years, but like then is still very much a non issue. With experience you can see when a box appears to be tampered with - if so, don't buy it. If you purchase it online, and it looks questionable - verify it.

If it makes you feel any better, please feel free to open all of your sets. I have also seen amazingly resealed plastic bags over the years - so you should also open all of those. The pieces could also be fake chinese ones (substituting the rare ones), so perhaps building the set might be the only way to know for sure that they are real LEGO (as the quality when built will be obvious - at least it will for now).

​I agree.  It is non issue for a regular LEGO set.  But if we are talking about dropping $500...$1000...$2000...or even $3000 on a set, I don't know...I wouldn't mind the seals being broken and the seller verifying the contents before purchasing the set to build.  It gets tricky when investors want to buy an expensive set to resell at a later time.  I think it will be up to us and other LEGO communities to say it is OK to open a set to confirm its contents, taking into consideration the fact that most secondary market sets are bought to build anyway.  

I think that's reasonable Ed. In fact I feel like I could list it was "Brand new, opened to verify contents and re-sealed" and it would still sell to the majority of people. On Amazon for sure. Ebay maybe a bit less.