Admirable though the maxim is generally, "A dog is for life" needn't always apply to sheepdogs"
This may seem a bad point to start discussing the negative side of training a sheepdog but it's worth bearing in mind that dogs are individuals, just as we are, and not all dogs get along with their owners!
Whilst I'm cautiously in favour of the ideal that a pet dog should be for life, certainly in the case of Border Collie sheepdogs, it isn't necessasarily true. I believe that if you really can't seem to make progress with a dog and you've taken expert advice and after genuine patient training over a reasonable period, still can't seem to make any headway, it's probably best to sell it.
Of course, you should be very careful who you sell it to - but I've sold dogs which I found difficult to train - and they've usually been better off for it. Some of them have gone on to make excellent sheepdogs and some went on to be much loved (and very happy) pets.
Likewise, I've taken on dogs which others have given up on - and they too, have blossomed as a result. This is because everyone's approach to training is different - and every dog's different. Quite frankly, there can be a personality clash between owner and dog. In an ideal world, we'd stick with our "problem" dog, learn why it's behaving the way it is (or isn't) and apply corrective measures but this isn't an ideal world. We don't all have the time it takes to learn every apect of canine psychology.
It's interesting to reflect that some people will change their live-in partner without regret but they'll stick with a dog that's causing them grief, through thick and thin!
If you do decide to part with your dog, it's extremely important to make certain the prospective buyer knows the dog's faults. Whenever I sell a dog, I try to make certain the buyer knows all the good and ALL the bad points about the animal.
On our sheepdogs for sale page, you'll notice that as well as the good points, I list the dog's faults so that the prospective buyer's aware of them right from the start. When the customer comes to see the dog, I demonstrate these good and bad points, and I show how to correct the faults. This way, the buyer won't be disappointed with the dog, so the dog gets off to the best possible start - the new owner's pleased with their purchase from day one.
Conceal the faults (quite easy if you're selling to someone less experienced than yourself) and when the dog and owner are out in the field, the new owner feels cheated. This is not conducive to good handler / dog relations and it certainly won't get you any recommendations!
Often, when I demonstrate a dog's bad points I find the buyer isn't bothered - "Oh, don't worry about that! As long as he'll get the sheep in, I'll be happy" is a common response! If the price is fair for the standard of work the dog's capable of, all should be well.