Ok so a sell order is one that you can wait and sell. It is the very least that you will accept for the coin. For example, if I have coin JLW and I have a sell limit order of 2.50. I will only accept that AND MORE. Nothing less.
Same with buy limit. If I have a buy limit of 2.49 for coin JLW I will never pay more for that.
No if I have 1000 JLW for sale, with a limit order of 2.50. Someone else could have a buy limit of 2.55 of JLW and 500. That order will fill, and then it will go on to the next order which is why your average price is not the same as your sell limit.
Now this did not include and take into account the First In First Out rule. FIFO is basically whoever was first has first dibs, then second, and contiunes as such.
A market order is so much faster, but yes, you may very well lose value on your coins becasue you are trying to sell it now to the best offer.