LOL welcome to sales! And it's not just snakes either.
Low-ballers abound everywhere. If you have a price in mind and you're not in a hurry to sell, then thank the buyer for his/her inquiry and state that the price is firm. If the offer is insultingly low just delete it and move on. After a few of these from the same person I might send a snarky response back. Example: snake is priced at $500, you get an offer of $100, you respond "Only if you pay in silver dollars."
If you don't want trades say so in the ad, usually something like "will trade only for pieces of paper with pictures of dead presidents on them, the power company doesn't want your ancient XBox" works.
Do not put a hold on anything without getting a nonrefundable deposit and a deadline for final payment. Example: 10% nonrefundable deposit, remainder due in 30 days, on day 31 the animal is back on the market.