I have a bunch there now but I got discouraged after looking into the keywording. They don't combine words, add/remove plurals, or automatically add media names (print, canvas etc). If you look at the recent sales you'll see most have added numerous combinations and variations of the main terms. For example you might see "london, bridge, tower, london bridge, london tower bridge, tower bridge, bridges, tower bridge print, london bridge print, london bridge canvas, london tower bridge photograph" and on and on. And then there's the plurals, where applicable.
I just looked a recently sold photo of a RAF Lancaster bomber. It included "Avro Lancaster Thumper, Lancaster Avro, Lancaster, Lancaster Bomber, Avro Lancaster Bomber, RAF Lancaster, RAF Lancaster Bomber, BBMF Lancaster, BBMF Lancaster Bomber" etc. and presumably the buyer searched on just one of those.
Doing all this for hundreds of photos is a big, tedious job and I'm not quite able to summon up the motivation. Like many people, my photos are already keyworded and I'm not going to mess up the IPTC for just one site, so I'd have to do it all during uploads. And of course if I did all that, the next week the site would announce that it's no longer necessary.
It also looks like the great majority of sales are landmarks, tourist sites, classic airplanes and ships, all UK subjects and I have none of that.
On the positive side, the people running the site seem reasonable and do answer questions.