Cobia and Matches already answered your question but I'll do so also.
Large corporate type auctions normally charge sales tax. Small units that use an auctioneer normally charge sales tax. However, not all facilities charge the sales tax. IMO it's stupid to pay tax on something that has already been taxed once but I degress.
Ask up front if they charges a "buyers premium", "sales tax", and "cleaning deposit". They should tell you all this before the auction, but I've been to a number that don't. Also be aware that the sales tax is AFTER the BP if there is one normally. So you can have these types of scenarios:
Figure $100 bid for a unit, 10% BP, 7% sales tax, $50 deposit (some charge this PER UNIT).
1) No deposit, tax, bp == $100
2) No deposit, w/ tax == $107
3) No deposit, w/ bp and tax == (100 * 10%) * 7% = $117.70
So if you get a facility that has all 3. You could be looking at $167.70 for a $100 unit till you get your deposit back. Here is a real world example from units buddy purchased the other week. This place charges a 10% bp, 7% tax, and $50 per unit cleaning deposit. He won 3 units that day.
Unit 1: $300 bid
Unit 2: $175 bid
Unit 3: $250 bid
He spent a total of $725 in bids + $72.50 bp + $55.83 (tax on bid + bp) + $150 (deposit) = $1003.33
Know the rules before you bid. I've seen a few newbies be surprised that the $150 locker they won cost them $176.55 instead of the $150 they "thought" they were paying.