Buying reused products rather than new ones could reduce the emissions of greenhouse gases (GHGs). There are many aspects that can influence whether used products actually cause emissions to increase or decrease. This paper assesses the effects of secondhand markets on GHG emissions by using data on twelve product categories from Troostwijk Auctions, which is a Dutch auctioning company. Data came from a carbon footprint database, survey data and many other sources. The net impact of secondhand trade is calculated by combining existing formulas in the literature and by performing regressions to estimate the values of unknown data. A methodology is proposed to find appropriate assumptions to handle uncertainty of carbon footprints. The main result is that the emission savings due to reuse of almost all products in the analysis are offset because many buyers purchase goods that they would not have bought new. Trade in the vehicles included in this study even adds emissions.