Actually, you could sustain it for at least 10 years without batting an eyelid.
More if you also include the negatives.
Well, this is one of the things where I have A LOTof experience having worked in the industry, if you can call it that.
Even if hundreds of competitors get in the market, that's not a problem for 2 reasons:
1) You can acquire them (and therefore get access to their photos)
2) The number of actual photos around is massive
3) The risk with types of photos is non-existent because collectors are of all
The only real issue is the categorization, let me show you an example:
imagine you have a photo of a panorama comprised of a tree in the foreground, a grassland with flowers, and a house in the back, yeah?
Great. That photo is that of a tree, but also that of a grassland. Then you can also say it's a photo of flowers, because they're visible in the picture.
Then it's also a photo of a house. D'oh! You see the problem there?
A tag cloud should solve that, but obviously you want to minimise the
roaming (even though collectors are used to it), so your website may be
helped by a local ranking per user of similar photos(FB released a machine learning library you can use for that). In other words, there are many ways to compete,
all you really need is a good idea of the industry, a decent website and time to scan the photos. However, you can minimise that by letting users upload the photos they want to sell. You have NO IDEA how much stuff there's out there.
However, for every investment you make, you quickly recoup and simply increase the profits :)
And, as I said, you can go on for years, because there's not just single photos, many families have very old albums that they don't value. Each album, on average, will contain 50 to 100 photos. There is just... there is just a mind boggling amount of money to be made.
And the stream really doesn't finish, because wives get mad, so you may end up acquiring (again) photos that you have already sold! LOL!
Obviously, you need to carefully target your price, but it can be done without a lot of hassle.