I find nearly all auctioneers, even small local ones, charge buyers premiums on all sales these days. Whether & how much additional they charge for remote bids(online/phone/left) is up to them and is required to be available in advance posted in their terms of sale.
Additional costs for on-line bidding: In many cases answer is yes depending on their own fee schedules (available on the auction house website) & those of the site who handles their online auctions. Depending on whether you are a recognized dealer or not and where the sale is being held you may also incur additional taxes/costs (city/county/state/ country, VAT, etc.)on the total bid. if the item would need to be imported additional customs/excise fees may apply.
I frequent some estate & antiques auctions near me for general merchandise as a private individual (as opposed to state registered reseller). One local auction house charges me 15% buyers premium on winning bids in person. That goes to 23% if I bid online (Phone and "left" bids may differ). They use one of the online auction/bidding providers for their online customers which I believe charges you an additional 3%-5% on the winning bid. add 8% local state sales tax if not a registered reseller (if NYC add city/state/etc to that).
Some auction houses may also have differing rules/fees on whether you pick-up the item yourself or have to shipped to you.
Even when bidding on-sight as a non-dealer for unrestricted items I have to consider roughly additional 24% to my order to keep in touch with reality.
A "great value" can easily add up to an unpleasant surprise if you don't practice "due diligence" by researching all rules/regulations/restrictions/conditions/costs in advance.
