How does one avoiding losing their shirts with auctions houses?

[Topnotchsy]

Edit: To make sure it's clear, I am not trying to bash the auction houses. I appreciate the service they provide. I just think I may be missing something here...

I'm somewhat new to this, so any pointers and advice would be greatly appreciated.

Over the last few years I've been buying items through some of the major auction houses. Many of the items I expect to keep for a long time, but recently I consigned a few items that were not central to my collection.

While I was thankfully able to cover the price I paid, this was what it looked like:

This was the numbers when I won the item:

Price $2,500.00
Buyer's Premium $487.50
Tax: 8.375%
Shipping: $22.05
Total $3,259.75

When I sold the item:

Price: $3,750
Buyer's Premium: $938
Tax: ?
Shipping: ?
Total: $4,688

What I netted:
Price: $3,750
Commission: $375 (10%)
Net: $3,375


I have never done this before but it was a little crazy seeing that too basically break even, the seller had to pay over $1400 more than I paid for the item (and that's without knowing if they paid sales tax or if what they paid for shipping.

I'm new at this and assume that there are ways to make this less extreme and was hoping for any pointers that people may have? Did I overpay/Should I have negotiated further with the auction house regarding the commission?

Thanks!