You have to understand what a bootleg is. In some cases genuine US releases were brought out in the UK in a legit way and would be collectable (Grapevine, Stateside, etc). There are also US reissues that are legit (MCA, Columbia SP, etc). In both cases the reissues would have only gone ahead by customer demand.
Then at the same time, came the illegal pressings, in some cases made to look identical to the real thing, and these flooded the scene in the 70s and 80s usually after a rare record had become a monster hit. Again customer driven because it was the only realistic way you could get it, unless you were rich. Most people on the scene then were not rich and so the pressings made alot of people happy and some dealers rich. Some DJs at that time may have helped create this situation.
25 years on, and telling whats real and genuine and whats not is indeed a skill known by a small few. Those who take collecting seriously. If you buy from The Manships and Soul Bowls of this world, it is probable that you will not get let down, but ebay is an auction and you pay before you receive, so watch out.
I think 90% of collectors have a made a mistake once or twice, so my advice is to send off for as many lists as possible and compare whats what, before you rush into buying. If you look at this forum (in your box) lots of readers will give advice on individual records.
Ed