I put in a bid for 500 shares on a NYSE stock at 1c above bid and got filled on 1 share. Why would the bot/market maker/whoever sell me one share? Is 500 shares some sort of threshold for something?
Quote from billyjoerob:
I put in a bid for 500 shares on a NYSE stock at 1c above bid and got filled on 1 share. Why would the bot/market maker/whoever sell me one share? Is 500 shares some sort of threshold for something?

Quote from Bob111:
i explained this many many times..it's done purposely and the reason for it to force you to buy at ask. i see it everywhere,tens times a day on every stock i trade. you got 1(2-3-5) share-you got a partial execution. now if you stay on bid -the price might(and most of the time will) move away and typically you ended up paying spread and double commission. or just buy at ask with pretty much same outcome..not sure,if it's really hard to understand the idea behind such fill. just sit down and think about possible outcome(we suppose to do this every time we trade anyway)![]()
Quote from abattia:
I can't find a reference to this anywhere, so it may be wrong (for which I will be justifiably flamed mercilessly, no doubt) but anyway ...
... to define the Best Bid and Offer, I believe there must be a total of at least a round lot of shares (i.e. a total of at least 100 shares, even if coming from multiple different orders) at the level.
If the above is correct, and you placed an order $0.01 above the bid...
... some other joker (or group of jokers, with orders together totalling less than a total of 100 shares) had resting limit orders below the ask. You couldn't see them as at each level they were for less than a total of 100 shares.
Your order got matched with the joker with a 1 share limit sell order resting $0.01 above the bid ...
... but then again, all the above could be wrong. Let's see what others think ...
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EDIT: OK, found it here:
https://www.federalregister.gov/art...ule-change-by-nyse-amex-llc-amending-its#p-26
"Interest will be quoted if it is equal to or greater than a round lot when the price point becomes the Exchange best bid and offer (âExchange BBOâ). "
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Why do people place 1 share limit orders? Because they are testing trading user interfaces, or an algo, or perhaps haven't a clue what they're doing and just saw some buttons they wanted to push? Who knows? Any of the above ...