Tips

Use several price comparison websites
Use several browswers
Leave the product in the shopping trolley for few days before finalising the order (because of the cookies, companies might send you reminders with offers or vouchers)
Check product reviews critically
Check sellers reviews crtically
Check information about the product you want and similar products (wikipedia, blogs, forums, browser, youtube, etc.)
Check the product you are about to buy carefully, as sometimes the reviews are for similar products with different specifications ( in a laptop could be different battery capacity, different processor, ram memory, software included, etc.)
Reviews can be used carefully, as sometimes can be subjective and suspicious. They can help also to find out more information about the product.
Check the different possibilities of delivery (in my case I prefer to collect it in Doodle, if possible, to minimise risk).
Use incognito or private browsing: Epicbrowser.com, duckduckgo.com Google incognito, Firefox private browsing, Opera private window.
In Amazon, consider the pros/cons of different sellers.
Camelcamelcamel.com can be useful to learn the prices that Amazon has displayed on a product at different times
Sometimes the reviews are of similar products that are not exactly the same.
As a general rule you have 14 days to return a product you have ordered by mistake (or because you have found it cheaper). In the case of CD/DVD they cannot be opened (outer plastic film must be intact).
Use different browsers as the mentioned above and Internet Explorer so the cookies don't overload one browser only.
If your computer is slow, use opera as it demands less resources.
Consider different options such as buying, renting or borrowing.
Consider buying a model from the previous year rather than the latest and check the specifications and prices. Sometimes an old version has some feature that the latest one lacks.
Use cashback sites such as www.topcashback.co.uk and www.quidco.com.
When buying from China (www.aliexpress.com) be careful with charges you might have to pay before collecting the items, days after shopping on-line.

Read the following about buying from China:
  www.moneysavingexpert.com/shopping/buy-from-china-aliexpress/

  www.theguardian.com/money/2017/sep/03/buying-online-china-aliexpress-beware-pitfalls

  www.makeuseof.com/tag/buy-safely-aliexpress-scams/

      https://www.bigcommerce.com/blog/alibaba-faqs-security-shipping-taxes/#exactly-how-to-use-alibaba-to-source-products

  https://andrewminalto.com/how-to-import-products-from-china/

www.bbc.co.uk/consumer/

www.moneysavingexpert.com/shopping/

www.savvywoman.co.uk

www.mrmoneymustache.com

www.dailyworth.com

www.topcashback.co.uk

www.quidco.com

GOOGLE CLEARANCE

https://www.myvouchercodes.co.uk

10 tips on paying off your debts

www.which.co.uk/money/money-saving-tips/getting-a-great-deal/haggling-acnth5v5q67h

https://www.which.co.uk/reviews/10-money-tips-for-parents/article/10-money-tips-for-parents

https://www.which.co.uk/money/money-saving-tips/saving-money/50-ways-to-save-money-a9g329r4s8hm

http://downloads.bbc.co.uk/tv/ripoffbritain/rip-off-britain-english.pdf

www.moneyadviceservice.org.uk/en

Pension calculators

Mortgage calculators

Credit cards calculators

Investments

SPANISH

Tips para comprar online

http://www.aecosan.msssi.gob.es/AECOSAN/web/home/aecosan_inicio.htm

www.kontsumobide.euskadi.eus/y10-kb2home/es/

www.consumer.es

https://www.ocu.org

http://www.eka.org

www.consumo-ccu.es

http://ahorradores.net

http://www.ahorristas.es

http://ahorrarencrisis.com

http://www.1000trucosparaahorrar.com

http://www.ahorratedinero.com

www.ahorro.net

https://www.comoahorrar.es

http://www.mediavida.com/foro/off-topic/mejores-paginas-webs-para-ahorrar-dinero-458569

http://www.lavanguardia.com/tecnologia/20160310/40346885485/webs-apps-espanolas-ahorrar.html

http://www.expansion.com/ahorro/2016/09/24/57e4f810468aebef1a8b45de.html