I'm all about saving money but when its your wedding gown, it may be worth it to just buy it from the bridal shop. I only say this because of the alterations. The only way I'd go with the ebay dress was if you know of a reputable seamstress who is experienced with wedding gowns, that you know personally who is going to do the alterations and do them for less than $300.
If you're gonna pay $545 for the dress & then get quotes of up to $300 for alterations, are you really saving a lot of $? And like you said, you're losing weight, etc, let the bridal shop do it & don't worry about the seamstress expense later. Does your dress have a lot of details & beading? Thats also something to think about.
I do like the idea of asking them to match the price.