Rating puzzles in order of difficulty is a very subjective. Everyone has different abilities when it comes to puzzle solving, but we've attempted to give relative difficulty ratings based on a combination of our own experience and watching others solving puzzles.
 
To explain how we rate puzzles:

Difficulty rating 1 are the easier puzzles to do and memorise. Difficulty rating 10 is the most difficult to do and memorise.
Puzzles rated 1, 2 or 3 may not take hours to do but there will always be some challenge involved. Don't write off these puzzles as too easy for adults.
For a puzzle to be rated a 9 or 10 it must be VERY difficult to complete the puzzle, and VERY difficult to memorise the puzzle's solution.
For example, a puzzle rated 7 could take an average person a couple of hours to do and then memorise.
A puzzle rated a 9 we expect that most people - say 80 % or so - will NEVER be able to master the puzzle without assistance from the solution.
How we present some of our puzzles will also have a bearing on the rating we give them. Some of our Interlocking puzzles are sold apart which adds the challenge of having to put the puzzle together without having first taken it apart. 
 
If you've given puzzles before and the person has always been able to do them quickly then you may wish to give them puzzles rated between 8 and 10. 
Whilst it is important to present the person with a challenge you don't want to discourage them by buying a puzzle they have no chance of ever doing.  So, if you're buying a person's first puzzle, but you still want it to be very hard, it may be wise to begin at a puzzle rated 5 to 7 rather than going for a 10 first up.

Of course, sometimes the most fun can be had with the easiest puzzles. Puzzlers always get a lot of enjoyment from sharing a seemingly simple puzzle amongst friends; the ones that you know you should be able to do but often take a little longer than expected (it always seems a lot longer when someone else is watching) so don't discard the idea of buying a level 1 or 2 puzzle out of hand.  You can get that  "Ah-Ha!" factor from them too.

DON'T DESPAIR.....  If all else fails the puzzles we sell include a Printed Solution sheet unless otherwise stated on the website.

Here at Mr Puzzle we not only sell but we also make under our own brand some of the most difficult puzzles in the world. Our burrs and even some of our other put-together puzzles are sold disassembled so that we can more accurately rate them because everybody needs to put them together without any possible assistance that taking the puzzle apart may have given.   Too see the range of Mr Puzzle's own brand click here for Standard, Craftsman or Limited Edition puzzles.