Well they come back every halloween so once the time comes, their value will go right down.
People will then make an excuse to value it really high because they're 'rare.'
Because of how easy it is to obtain a pumpkin when they were originally available (farmville rp xd) they aren't really that valuable. Generally they should be sold at around 1.5k - 4k meanwhile, the watermelon due to it's superior rarity because it's near impossible to obtain (admin spawning presents) would be priced around 70k range.
Experience: used to run Antiques Roadshow in Paralake.