No idea about contract legalities, but very poor form and I sympathise with your annoyance,. My thoughts, in no particular order.
How do you forget you have an item on your website when you list it on ebay and not remember to check what you were asking?
I have always found that dealers are quite happy to offer items for less off ebay as it reduces their costs, so I'd at least anticipate a reduction from the £3500!
The longer a dealer has an item on their site unsold the more I would expect them to discount it! Unsold = no profit and funds tied up. And if it hasn't sold at the web price, they're just chancing it with hiking it that much on ebay!
And this particular case, I'm guessing;
A: Like in a shop that has forgotten to re-price an item, they aren't obliged to sell to you. But until it sells at the higher price its value is not established. If I'd already paid, I'd just ask for my money back. In six months time, if it hasn't sold and you still want it, make an offer. Unless you really want it and think the price is still reasonable. I'd still haggle as they've avoided eBay costs and fees. 20% down from the ebay price might be a starting point?
B: No contract has been entered into and the price is open to negotiation. I suspect that's always the case, just that many retailers will honour an incorrect lower price if it isn't hugely out to preserve customer goodwill. Though as the economy gets worse, just like John Lewis and their 'never knowingly undersold' promise, I imagine many will review this if they haven't already.