Well the thing is, not too many know about Shenmue. Lets say 30 people are collecting stuff (games mostly).
Inside this groupe, you'll have like 20 people trying to get some goods, vhs tape, whats shenmue demo.
Once again, within them, you'll have 10 people trying to get more goodies because they care more : cards, guide book
And the left ones are mostly the most interested people, out of 10, you'll probably find 5 people going as far as buying mugs, mousepad at very high price. Some looking for old items like your artbook from tgs 99 etc.
The thing is, once the rich ones (well rich... i mean people having a lot of money), you wont have many people left intested in buying, or atleast not at high prices.
5 people looking to buy one artbook that comes out from time to time, boom, one will sell for like 300 euros. Once 4 of them have it, the last one will most likely not sell around that price.
Its just like the timex watch. I mean, who, on the shenmuedojo is willing to pay more than 1000 dollars for a watch ? Considering the fact that if you erase "shenmue" from it, it cost 40 $.... nothing more, probably not that many.
So yeah, if we manage to see another two or 3 timex watch on ebay in the next few years, the last one might drop in price (unless the same rich guy plan to buy more than one of each item, then again i wouldn't understand why). I mean, having 10 copies of shenmue if you pay it 10 euros, why not. But paying 5000$ to have 5 watch, that's stupid
Now about the watch, its up to you. It seems like you have 3-4 people interested here (me included). So you can either sell it to the highest offer (i'd seriously offer a lot but right now i am short, if it was the right moment, i'd say 800$), or you can sell it on ebay, but then again, that's up to you !