I've agreed to a buyers offer to purchase a vehicle that I have listed on CarGurus. I've been unable to get a response from the buyer for several days now. Is there a process to proceed to the next buyers offer to purchase this same vehicle since the previous buyer has not responded to complete the transaction?

I have some experience with contracts and would advise that you notify the reticent Buyer and let them know that if they don't respond within a day then you will have to abandon the offer of sale. This isn't necessarily required - you could just ignore them and move on to the next customer - but it is polite and helps avoid any potential confusion regarding implied intent to proceed, etc.