I wouldn't do that if I were you. The problem is that just because somebody else did a Purchase Offer already does not mean he will end up getting it. The position of your transaction in the block could still be above the one send earlier.
It doesn't necessarily have to be a hide, it can be a highlight/color change etc - just something to let the user know that there is already an accept visible on the blockchain for this sell and thus their accept is likely to be invalidated. You're correct in that there is nothing stopping the user (protocol wise) from sending their own accept and hoping it gets included earlier in the block than the accept already visible.
That's essentially the crux of the scalability problem - an exchange where the lowest ask sell can only be bought once every (on average) 10 minutes just doesn't scale. To avoid everyone trying to accept the same sell we need a way to consider that lowest sell 'pending acceptance' (or 'Purchase Offered') and have users look to the next lowest sell intra-block to make it scalable in my opinion.
A color indication would be a great idea. I personally would still try to get the cheapest option since I have just as much chance of getting it then the person who already submitted an offer. All I wanted to convey is that the first message seen by a certain node has no increased chance of being the first one in the block. I would prefer the cheapest option, even if I need to fight for it. But by coloring it you at least give people an option to decide what is important for them.
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