This dissertation contains two essays focusing on how Self and Competitor Expertise influence valuation both during and post-auction. The first essay, "Competitor Expertise: Influences on bidding behavior and post-auction values in ascending auctions," considers how a bidder's perception of competitors' expertise types and levels influences valuation both during (bid level), after (WTP/WTA), and over time (∆WTP/∆WTA). Generally, I find that despite normative predictions regarding bidding behavior in a competitive auction environment, bidders tend to bid higher and maintain higher post-auction valuations when competing against experts in the product domain, although not amateurs or experts in other domains (e.g., auction bidding strategies). Post-auction valuation patterns further depended on Auction Outcome. The second essay, "Assessed Self-Expertise: Influences on Bidding Behavior and Post-Auction Values Against Competitors of Varying Expertise Levels," extends our investigation to consider how a bidder's perception of their own expertise type and level also influences valuation both during and post-auction. Broadly, I find additional support that bidders post higher valuations both during and post-auction when competing against Experts vs. Amateurs, but that this behavior is primarily driven by bidders who assess themselves as Experts and further depends on Auction Outcome.